Upstream suggested that the --with-mpi-dir configuration options does not need to be (and should not be) invoked if compilation is done with mpicc and friends. And use of mpicc is recommended to get the flags right.
The short workaround for you is to set mpicc as your CC. mpicc sets the include dirs that you're missing. Let me know if that fits in with the intentions of the PETSc developer who made the FindPETSc.cmake that you're using. If not, we can discuss it further with them and the other PETSc developers. Drew
