meson: unify linux/stat.h check with other checks and use _GNU_SOURCE

Using _GNU_SOURCE is better because that's how we include the headers in the
actual build, and some headers define different stuff when it is defined.
sys/stat.h for example defines 'struct statx' conditionally.

src/basic/missing.h

@@ -24,7 +24,7 @@
 #include <uchar.h>
 #include <unistd.h>
 
-#if !HAVE_STRUCT_STATX_IN_SYS_STAT_H
+#if WANT_LINUX_STAT_H
 #include <linux/stat.h>
 #endif