Please always follow these steps:

[X] Confirmed this is a problem with brew installing one, specific Homebrew/homebrew-core formula (not cask or tap) and not every time you run brew? If it's a general brew problem please file this issue at https://github.com/Homebrew/brew/issues/new. If it's a brew cask problem please file this issue at https://github.com/caskroom/homebrew-cask/issues/new. If it's a tap (e.g. Homebrew/homebrew-php) problem please file this issue at the tap.
[X] Ran brew update and retried your prior step?
[X] Ran brew doctor, fixed all issues and retried your prior step?
[X] Ran brew gist-logs <formula> (where <formula> is the name of the formula that failed) and included the output link?
[ ] If brew gist-logs didn't work: ran brew config and brew doctor and included their output with your issue?

The aqbanking build is broken on 10.11, 10.12, and 10.13 for me. I don't think it's a High Sierra specific problem even though it's listed in #18493.

gist-logs from 10.12: https://gist.github.com/05f135e7c543ca2ac11838500be2c4a8

The error:

$ brew install -s aqbanking
==> Downloading https://www.aquamaniac.de/sites/download/download.php?package=03&release=208&file=01&dummy=aqbanking-5.6.12.tar.gz
Already downloaded: /Users/janke/Library/Caches/Homebrew/aqbanking-5.6.12.tar.gz
==> ./configure --prefix=/usr/local/Cellar/aqbanking/5.6.12 --enable-cli --with-gwen-dir=/usr/local
==> make check
Last 15 lines from /Users/janke/Library/Logs/Homebrew/aqbanking/02.make:
libtool: compile:  clang -DHAVE_CONFIG_H -I. -I../../../.. -I../../../../aqbanking5 -DBUILDING_AQBANKING -g -O2 -Wall -c accstatus.c  -fno-common -DPIC -o .libs/accstatus.o
In file included from accstatus.c:19:
In file included from ./accstatus_p.h:18:
In file included from ./accstatus_l.h:18:
In file included from ../../../../aqbanking5/aqbanking/accstatus.h:18:
../../../../aqbanking5/aqbanking/value.h:18:10: fatal error: 'gwenhywfar/buffer.h' file not found
#include <gwenhywfar/buffer.h>
         ^~~~~~~~~~~~~~~~~~~~~
1 error generated.
make[5]: *** [accstatus.lo] Error 1
make[4]: *** [check-recursive] Error 1

But if you look for that header file, it looks like it's under a gwenhywfar4 subdirectory.

[/usr/local/opt/gwenhywfar]
$ find . -name buffer.h
./include/gwenhywfar4/gwenhywfar/buffer.h

Perhaps the layout of gwenhywfar changed in one of its recent updates since the last aqbanking update (Oct 11 2016).

Anybody know enough German to report this bug upstream? Though it looks like their Trac is down at the moment.

Something goes wrong during configure with Xcode 9

checking for library containing gettext... -lintl
checking whether this is an official release... no
checking for compiler arguments... -g -O2 -Wall
checking whether compiler supports symbol visibility... no
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
checking docpath... /private/tmp/aqbanking-20180210-37246-1hbnm4v/aqbanking-5.6.12/.brew_home/apidoc
checking if full docu should be created... no
checking doxygen tag files... none
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character rangesed: 
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range1: "s;-I[-Wunused-command-l ...": RE error: invalid character range

sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character rangesed: 
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
checking aqhbci data dir... 
sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character rangesed: 
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range1: "s;-I[-Wunused-command-l ...": RE error: invalid character range

sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range1: "s;-I[-Wunused-command-l ...": RE error: invalid character range

sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character rangesed: 
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range1: "s;-I[-Wunused-command-l ...": RE error: invalid character range

checking aqebics data dir... 
checking for build requirements needed by EBICS backend... 
checking for XMLSEC... yes
checking for LIBXML... yes
checking for LIBXSLT... yes
checking if plugin init is wanted... yes
checking if AqBanking is a subproject... no
checking whether aqbanking-cli is wanted... yes
checking whether tutorials are to be built... yes
sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character rangesed: 
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character rangesed: 
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
sed: sed: 1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
1: "s;-I[-Wunused-command-l ...": RE error: invalid character range
checking that generated files are newer than configure... done
configure: creating ./config.status

The same doesn't happen on 10.11.

The difference in the resulting config.status is

< S["all_includes"]="-I/usr/local/Cellar/gwenhywfar/4.18.0/include/gwenhywfar4"
---
> S["all_includes"]=""

So we can

    ENV["CPPFLAGS"] = "-I#{Formula["gwenhywfar"].opt_include}/gwenhywfar4"

and kick the issue upstream.

Homebrew / homebrew-core

aqbanking build is broken #23973

Please always follow these steps: