Autogen.sh fails due to CPPFLAGS_FOR_BUILD, CFLAGS_FOR_BUILD

I use source code of ElonCoin Core for Linux from this repository. But it does not build.

# ./autogen.sh
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, 'build-aux'.
libtoolize: copying file 'build-aux/ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'build-aux/m4'.
libtoolize: copying file 'build-aux/m4/libtool.m4'
libtoolize: copying file 'build-aux/m4/ltoptions.m4'
libtoolize: copying file 'build-aux/m4/ltsugar.m4'
libtoolize: copying file 'build-aux/m4/ltversion.m4'
libtoolize: copying file 'build-aux/m4/lt~obsolete.m4'
configure.ac:45: installing 'build-aux/compile'
configure.ac:28: installing 'build-aux/missing'
Makefile.am: installing 'build-aux/depcomp'
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, 'build-aux'.
libtoolize: copying file 'build-aux/ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIRS, 'build-aux/m4'.
libtoolize: copying file 'build-aux/m4/libtool.m4'
libtoolize: copying file 'build-aux/m4/ltoptions.m4'
libtoolize: copying file 'build-aux/m4/ltsugar.m4'
libtoolize: copying file 'build-aux/m4/ltversion.m4'
libtoolize: copying file 'build-aux/m4/lt~obsolete.m4'
configure.ac:10: installing 'build-aux/compile'
configure.ac:9: installing 'build-aux/missing'
Makefile.am:153: error: CPPFLAGS_FOR_BUILD must be set with '=' before using '+='
Makefile.am:154: error: CFLAGS_FOR_BUILD must be set with '=' before using '+='
Makefile.am: installing 'build-aux/depcomp'
autoreconf: automake failed with exit status: 1

Machine specs:

OS: Devuan beowulf, Linux devuan 4.19.0-14-amd64 #1 SMP Debian 4.19.171-2 (2021-01-30) x86_64 GNU/Linux
CPU: Intel(R) Pentium(R) CPU N3530 @ 2.16GHz
RAM: 8 Gb
Disk size: 50Gb
Disk Type (HD/SDD): SSD

autoconf --version autoconf (GNU Autoconf) 2.69 gcc --version gcc (Debian 8.3.0-6) 8.3.0 make --version GNU Make 4.2.1

EMCoin / Eloncoin

Autogen.sh fails due to CPPFLAGS_FOR_BUILD, CFLAGS_FOR_BUILD #3

Machine specs: