./configure incorrectly adds mmacosx-version-min to iOS/watchOS/tvOS builds

[hamstergene]

Problem

./configure script contains this piece:

  19975     if test -z "$(echo $CFLAGS | grep m.*os.*-version-min)"; then
  19976       min="-mmacosx-version-min=10.8"                                                                                                   
  19977       CFLAGS="$CFLAGS $min"

It breaks iOS/watchOS/tvOS (any non-macOS) build if -miphoneos-version-min is given as part of CC rather than part of CFLAGS for example:

CC=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc  -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS13.2.sdk -miphoneos-version-min=9.0
CFLAGS=-g -fPIC

...or if the minimum version is provided via an environment variable instead:

export IPHONEOS_DEPLOYMENT_TARGET=9.0

What I expected

1. curl configure script either needs better detection of *-version-min presence
2. or maybe it needs to lose that mmacosx-version-min piece and let the user take care of that

Context

The reason I tried providing essential flags via CC instead of CFLAGS is #3189 — the true cause of that bug was that ./configure was invoking preprocessor as $CC -E without the essential -isysroot flag which caused some autoconf checks to fail to find any system headers. The advice inside #3189 to update command line tools is not correct way to do it; the correct solution is to give sysroot/target/etc to both CFLAGS, CPPFLAGS, and LDFLAGS.

[bagder]

Maybe we could just extend the check and also see if the user has set the option in CC? Like this:

diff --git a/acinclude.m4 b/acinclude.m4
index e7a36e4bd..f78bb547d 100644
--- a/acinclude.m4
+++ b/acinclude.m4
@@ -2524,13 +2524,13 @@ AC_DEFUN([CURL_MAC_CFLAGS], [

   AC_MSG_CHECKING([for good-to-use Mac CFLAGS])
   AC_MSG_RESULT([$tst_cflags]);

   if test "$tst_cflags" = "yes"; then
-    AC_MSG_CHECKING([for *version-min in CFLAGS])
+    AC_MSG_CHECKING([for *version-min in CFLAGS or CC])
     min=""
-    if test -z "$(echo $CFLAGS | grep m.*os.*-version-min)"; then
+    if test -z "$(echo $CFLAGS $CC | grep m.*os.*-version-min)"; then
       min="-mmacosx-version-min=10.8"
       CFLAGS="$CFLAGS $min"
     fi
     if test -z "$min"; then
       AC_MSG_RESULT([set by user])

[bagder]

We could perhaps even check $IPHONEOS_DEPLOYMENT_TARGET in there?

[hamstergene]

Grepping $CC $CFLAGS should cover most users I think. I don't know how many users use environment variables, but on my memory I can't recall seeing that in the wild often.

Checking environment may be a bit more complicated:

  % strings /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang | grep DEPLOYMENT_TARGET
MACOSX_DEPLOYMENT_TARGET
IPHONEOS_DEPLOYMENT_TARGET
TVOS_DEPLOYMENT_TARGET
WATCHOS_DEPLOYMENT_TARGET
BRIDGEOS_DEPLOYMENT_TARGET
DRIVERKIT_DEPLOYMENT_TARGET

EDIT: the problem with environment variables is that we don't know if they will actually be used. Maybe IPHONEOS_DEPLOYMENT_TARGET is set, but the compiler will be targeting Mac? IMO that's bad design from Apple side, there is just no way for curl to have it 100% right in all cases.