meson: GNU/kFreeBSD has DRM/KMS and requires -D_GNU_SOURCE

This is a regression from the old autotools build system.

Full GNU/kFreeBSD support is blocked on fixing drm.h, and then it dies late in the build because libsync.h wants ETIME which does not exist on plain FreeBSD nor on GNU/FreeBSD and this is only patched downstream. Nonetheless, this merge request gets mesa itself ready for when these are fixed, as it builds successfully if I also patch those files.