Log Message:
devel/SDL2/buildlink3.mk: Expose sdl2-config to cross-builds.

Although sdl2-config is part of the cross-built package, we execute
it out of the cross-destdir nevertheless because it's a shell script
and it provides the necessary configuration relative to the sysroot
for building dependencies.

No change to native builds because this is conditional on
USE_CROSS_COMPILE.