glew/cross: Fix building with mingw-w64.

This also cleans up the package expression a bit and we no longer
directly override the phases (except patchPhase).

Also for cross-builds we're using the stock mingw target and pass CC, LD
and STRIP to make, because the mingw-w64 targets of the upstream package
are outdated.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>