graphics/mesa: Fix minor issues from meson switch

Fix a couple of minor issues with mesa related ports after the switch to
meson.
Define _GNU_SOURCE when building on FreeBSD. This fixes blur effects in
Plasma 5 and issues with the SDDM greeter. This fix was originally applied
in r516100 but got lost in the switch to meson. [1]
Properly detect sys/sysctl.h on FreeBSD. On FreeBSD sys/sysctl.h needs
sys/types.h as well, patch the meson build script to take this into account.
Fix a broken sed command to properly set ${LOCALBASE}/etc and
${LOCALBASE}/share/ as appropriate.

Bump portrevision of all mesa related ports.

PR: 247138 [1]
Reported by: Roman [1]
Reviewed by: manu
Tested by: Roman, Serge Volkov
Differential Revision: https://reviews.freebsd.org/D25265