math/suitesparse: fix build on powerpc64 elfv1

After last update, compiler:openmpi comes before compiler:c++11-lang. This effectively overrides compiler:c++11-lang and causes base GCC to be used.
compiler:openmpi could probably be dropped, now that all supported FreeBSD versions have compiler that supports OpenMP (either GCC or Clang), but this would mean that Clang architectures would switch from GCC to Clang for this port, so it would have to be separately tested by the maintainer.