build: tolerate use of _Generic from glibc 2.43 with Clang (#6233)

The `linux-{asan-ubsan,sha256,reftable}` jobs run inside
`ubuntu:rolling`, which now resolves to Ubuntu 26.04 with glibc 2.43;
that pulls `_Generic` into `<sys/cdefs.h>` and breaks our `-std=gnu99
-Werror` Clang builds. Concrete failure:
https://github.com/git-for-windows/git/actions/runs/25390480083/job/74463338845.

Picking up Patrick Steinhardt's fix from
https://lore.kernel.org/git/20260505-b4-pks-ci-tolerate-glibc-generic-v1-1-5786386fe512@pks.im/
ahead of its upstream merge so the GfW CI goes green again. The diff
conflicts with `fe5704a3695c "mimalloc: offer a build-time option to
enable it"`, which wraps the affected `config.mak.dev` block in `ifndef
USE_MIMALLOC`; the resolution preserves that wrap on the `gcc6`-only
branch surviving Patrick's patch. `meson.build` auto-merged.

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

meson.build

@@ -867,6 +867,12 @@ if get_option('warning_level') in ['2','3', 'everything'] and compiler.get_argum
       libgit_c_args += cflag
     endif
   endforeach
+
+  # Clang generates warnings when compiling glibc 2.43 because of the use of
+  # _Generic.
+  if compiler.get_id() == 'clang'
+    libgit_c_args += '-Wno-c11-extensions'
+  endif
 endif
 
 if get_option('breaking_changes')