When I added the option for compiling with openmp, like other ports, I don't use compiler:openmp but directly the default compiler because it works just as well. However, on FreeBSD 11, grass only builds well with LLVM90.
While I'm here I added with-proj-[includes|libs] to be more thorough (There's a problem on 13i386 that I still can't solve properly) and I remove useless post-patch commented.