Install and use xorgproto, and build with optimization as clang has
warnings that only work above -O0. Also expand the build matrix for both
Xcode 8.3 and 9.2. Both images are OSX 10.12 at the moment so this is
really just a way to test with clang 8 vs 9.
Signed-off-by: Adam Jackson <ajax@redhat.com>