databases/grass7: fix build on 11i386

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

Approved by: tcberner
MFH: 2020Q3
Differential Revision: https://reviews.freebsd.org/D25712