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

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.

(cherry picked from commit 9c869d08d82c73f62ab3527567858ce4b0cf1257)

Gbp-Pq: Name meson-unify-linux-stat.h-check-with-other-checks-and-use-.patch