build: bring the _Generic-from-glibc-2.43 patch in line with upstream (#6242)

Upstream has landed Patrick Steinhardt's patch [build: tolerate use of
_Generic from glibc 2.43 with
Clang](https://lore.kernel.org/git/xmqqecjischc.fsf@gitster.g) in a form
that differs from what we currently carry as 74c3ea9 (a cherry-pick
of Patrick's v1). The upstream variant on `next` (commit 0a6d290)
carries the trailer `[jc: replaced Makefile side with Shardul's
approach]` and is signed off by Junio.

Concretely, instead of dropping `-std=gnu99` for Clang in
`config.mak.dev`, the upstream variant keeps `-std=gnu99` and adds
`-Wno-c11-extensions` for Clang in a dedicated top-level block,
mirroring what Patrick already did on the Meson side. Keeping
`-std=gnu99` is desirable because it still lets us catch accidental use
of newer-than-C99 features in Git's own code.

This PR is the amend! commit that brings our downstream pick in line
with that upstream landing. Autosquashing onto the parent of 74c3ea9
produces a single commit whose tree state is identical to 0a6d290
(modulo our USE_MIMALLOC carry inside the gcc6/clang7 block on the
Makefile side). The intended merge strategy is
autosquash-then-force-push to `main`, not a regular merge commit, so the
history mirrors what landed upstream.

Junio's confirmation that he is fast-tracking the topic to `next` and
`master` is at https://lore.kernel.org/git/xmqqecjischc.fsf@gitster.g.

Author: dscho

config.mak.dev

@@ -21,15 +21,12 @@ endif
 endif
 
 ifneq ($(uname_S),FreeBSD)
-ifneq ($(filter gcc6,$(COMPILER_FEATURES)),)
+ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang7,$(COMPILER_FEATURES))),)
 DEVELOPER_CFLAGS += -std=gnu99
 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
@@ -101,6 +98,13 @@ endif
 endif
 endif
 
+# glibc 2.43 headers unconditionally use _Generic even when we ask the
+# compiler to stick to -std=gnu99 and unlike GCC, clang lacks a
+# workaround to squelch warnings from system headers.
+ifneq ($(filter clang1,$(COMPILER_FEATURES)),)     # if we are using clang
+DEVELOPER_CFLAGS += -Wno-c11-extensions
+endif
+
 # https://bugzilla.redhat.com/show_bug.cgi?id=2075786
 ifneq ($(filter gcc12,$(COMPILER_FEATURES)),)
 DEVELOPER_CFLAGS += -Wno-error=stringop-overread