sparse: ignore warning from new glibc headers

With at least glibc 2.39, glibc provides a function declaration that
matches with this POSIX interface:

    int regexec(const regex_t *restrict preg, const char *restrict string,
           size_t nmatch, regmatch_t pmatch[restrict], int eflags);

such prototype requires variable-length-array for `pmatch'.

Thus, sparse reports this error:

> ../add-patch.c: note: in included file (through ../git-compat-util.h):
> /usr/include/regex.h:682:41: error: undefined identifier '__nmatch'
> /usr/include/regex.h:682:41: error: bad constant expression type
> /usr/include/regex.h:682:41: error: Variable length array is used.

Note: `__nmatch' is POSIX's nmatch.

The glibc's intention is informing their users to provides a large
enough buffer to hold `__nmatch' results and provides diagnosis if
necessary.  It's merely a glibc' implementation detail.

Hide that usage from sparse by using standard C11's macro:
__STDC_NO_VLA__

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: sgn

Makefile

@@ -1409,7 +1409,7 @@ ARFLAGS = rcs
 PTHREAD_CFLAGS =
 
 # For the 'sparse' target
-SPARSE_FLAGS ?= -std=gnu99
+SPARSE_FLAGS ?= -std=gnu99 -D__STDC_NO_VLA__
 SP_EXTRA_FLAGS =
 
 # For informing GIT-BUILD-OPTIONS of the SANITIZE=leak,address targets