fix(msan): pin _GNU_SOURCE for the zstd object alongside the forced stdint

Scoping -include stdint.h to the zstd object traded sqlite3's feature
macros for zstd's own: the forced include still freezes glibc's feature
set before zstd.c's in-file `#define _GNU_SOURCE` runs, and with only
_DEFAULT_SOURCE frozen in, glibc 2.39 does not declare qsort_r --
zstd.c:47409 fails exactly as the x86-64 leg reported. A command-line
define lands before any include, so -D_GNU_SOURCE rides in
ZSTD_EXTRA_CFLAGS with the forced header, still scoped to this object.

Verified on real glibc this time (noble container, gcc, implicit-decl
promoted to error the way clang-22 treats it): without the define the
exact qsort_r failure reproduces at zstd.c:47409; with it the file is
clean. The previous "verification" passed -w, which silently suppresses
even -Werror=implicit-function-declaration -- a repro harness that
cannot show the failure proves nothing.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

scripts/msan.sh

@@ -46,7 +46,12 @@ MSAN_SAN="-fsanitize=memory $MSAN_ORIGIN_FLAG -fno-omit-frame-pointer -isystem $
 # Makefile.cbm): zstd's MSan block needs stdint.h that its amalgamation lost,
 # but force-including it globally freezes glibc feature-test macros before
 # sqlite3.c can set _GNU_SOURCE, breaking that compile instead.
-ZSTD_EXTRA="-include stdint.h"
+#
+# -D_GNU_SOURCE rides along for the same freeze reason IN this object:
+# a command-line define lands before the forced include, while zstd.c's own
+# in-file feature setup lands after it -- without this, glibc freezes
+# without _GNU_SOURCE and zstd loses qsort_r.
+ZSTD_EXTRA="-D_GNU_SOURCE -include stdint.h"
 
 # Always clean: make does not encode flags into dependencies, so a build dir
 # populated under different stdlib/sanitizer flags silently mixes objects