graphics/mesa: Fix pr247138, detect sys/sysctl.h and fix sed

[zeising]

Summary

This is a round up of misc fixes to mesea ports:
Fix pr247138 by properly defining _GNU_SOURCE when building on FreeBSD. This was lost when moving from autotools to meson
Properly detect sys/sysctl.h on FreeBSD, it requres sys/types.h.
Fix a broken sed command to properly set ${PREFIX}/etc and ${PREFIX}/share as appropriate.

Test Plan

Build and see that -D_GNU_SOURCE and -DHAVE_SYS_SYSCTL_H is set on the command line.
Verify that the sed command actually worked by checking the resulting src/util/xmlconfig.c .

[manu]

Thanks for fixing thoses issues.