build: Define _GNU_SOURCE for gmtime_r

Found while building tracker 3.5.2 with clang 16, which enable
Wimplicit-function-declaration by default.

Without _GNU_SOURCE defined, tracker would fail in the configure phase
with Checking if "strftime 4-digit year modifier" runs: DID NOT
COMPILE error. And investigating the meson log we see

Compiler stderr:
 /var/tmp/portage/app-misc/tracker-3.5.2/work/tracker-3.5.2-build/meson-private/tmplr2vm6b0/testfile.c:12:5: error: call to undeclared function 'gmtime_r'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    gmtime_r (&timestamp, &tm);

And a little further up

Command line:  clang /var/tmp/portage/app-misc/tracker-3.5.2/work/tracker-3.5.2-build/meson-private/tmplr2vm6b0/testfile.c -o /var/tmp/portage/app-misc/tracker-3.5.2/work/tracker-3.5.2-build/meson-private/tmplr2vm6b0/output.exe -O2 -pipe -march=native -DTRACKER_DEBUG -DG_DISABLE_CAST_CHECKS -D_FILE_OFFSET_BITS=64 -O0 -Werror=implicit-function-declaration -std=c99 -Wl,-O1 -Wl,--as-needed -fuse-ld=lld -rtlib=compiler-rt -unwindlib=libunwind -Wl,--as-needed

Seems like cc.run() doesn't pick up the general compiler args we set
earlier. We have already set it for the main build, so adding it just
for the configure test should be fine.

Signed-off-by: Brahmajit Das <brahmajit.xyz@gmail.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/meson.build b/meson.build
index 7b22cb1f9..00af07e6e 100644
--- a/meson.build
+++ b/meson.build
@@ -202,6 +202,7 @@ endif
 # Get an appropriate 4-digit year modifier for strftime
 ##################################################################
 result = cc.run('''
+  #define _GNU_SOURCE
   #include <stdio.h>
   #include <string.h>
   #include <time.h>