Import upstream fix to stop sdl-config from unconditionally defining
_GNU_SOURCE.

PR: 198745
Submitted by: me
Differential Revision: https://reviews.freebsd.org/D2066
Approved by: mva@ (maintainer)
Reported by: Georg Kilzer (leper)
Obtained from: http://hg.libsdl.org/SDL/rev/7b87c1773b3e