add generic Mac toolchain file

I prefer that CMake find the MPI compiler wrappers instead of the base
compilers, if for no other reason than this often means mixing clang and
gfortran, since clang will likely be found first in PATH.

this toolchain assumes MPI wrappers are in the PATH but one can set the
base directory (with a trailing "/") if desired.

the BLAS/LAPACK used is "-framework Accelerate"