build: tolerate use of _Generic from glibc 2.43 with Clang

When building with `make DEVELOPER=1` we explicitly pass "-std=gnu99" to
the compiler so that we don't start leaning on features exposed by more
recent versions of the C standard. Unfortunately though, glibc 2.43
started to use type-generic expressions. This works alright with GCC,
but when compiling with Clang this leads to errors:

  $ make DEVELOPER=1 CC=clang
  CC daemon.o
  In file included from daemon.c:3:
  ./git-compat-util.h:344:11: error: '_Generic' is a C11 extension [-Werror,-Wc11-extensions]
    344 |         return !!strchr(path, '/');
        |                  ^
  /usr/include/string.h:265:3: note: expanded from macro 'strchr'
    265 |   __glibc_const_generic (S, const char *, strchr (S, C))
        |   ^
  /usr/include/x86_64-linux-gnu/sys/cdefs.h:838:3: note: expanded from macro '__glibc_const_generic'
    838 |   _Generic (0 ? (PTR) : (void *) 1,                     \
        |   ^

In theory, the `__glibc_const_generic` macro does have feature gating:

  #if !defined __cplusplus \
      && (__GNUC_PREREQ (4, 9) \
          || __glibc_has_extension (c_generic_selections) \
          || (!defined __GNUC__ && defined __STDC_VERSION__ \
              && __STDC_VERSION__ >= 201112L))
  # define __HAVE_GENERIC_SELECTION 1
  #else
  # define __HAVE_GENERIC_SELECTION 0
  #endif

But this feature gating isn't effective because `_has_extension()` will
always evaluate to true as C generics _are_ available as a language
extension to GNU C99 when using Clang. This would have been different if
`_has_feature()` was used instead, in which case it would have properly
evaluated to `false`.

Unfortunately, there is no easy way for us to work around the warning.
We cannot define `__HAVE_GENERIC_SELECTION` ourselves as that would lead
to a redefinition, and given that the conditions are or'd together we
cannot disable any of those, either.

Instead, work around the issue by not using -std=gnu99 with Clang when
using the Makefile and by disabling warnings about C11 extensions when
using Meson. This isn't ideal, but we at least retain the ability to
detect the (mis-)use of features from newer standards with GCC.

An alternative to this might be to simply bump the required C standard
to C11, which is 15 years old by now and should have support on most
platforms out there. But some more esoteric platforms may not have it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

config.mak.dev

@@ -21,7 +21,7 @@ endif
 endif
 
 ifneq ($(uname_S),FreeBSD)
-ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang7,$(COMPILER_FEATURES))),)
+ifneq ($(filter gcc6,$(COMPILER_FEATURES)),)
 ifndef USE_MIMALLOC
 DEVELOPER_CFLAGS += -std=gnu99
 endif
@@ -29,6 +29,9 @@ endif
 else
 # FreeBSD cannot limit to C99 because its system headers unconditionally
 # rely on C11 features.
+#
+# Clang cannot limit to C99 when using glibc 2.43 because its system headers
+# depend on the _Generic C11 feature. This works with GCC though.
 endif
 
 DEVELOPER_CFLAGS += -Wdeclaration-after-statement