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`.

GCC has a workaround to squelch this warning from standard system
headers, but because clang fails due to [-Werror,-Wc11-extensions],
as it lacks the corresponding workaround.

For both meson and Makefile, pass -Wno-c11-extensions when we are
building with clang.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Helped-by: Shardul Natu <snatu@google.com>
[jc: replaced Makefile side with Shardul's approach]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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