Linking with external requires no-serial option

[sshiraiwa]

When building PyMFEM using --mfem-prefix to link the wrapper with MFEM which was built separately outside PyMFEM setup.py, the build could fail during the serial wrapper building.

Reason:
Serial wrapper (mfem.ser) assumes that mfem is built without any dependency.

Short-term solution:
add -no-serial option to skip building the serial wrapper.

Long-term solution:
expand mfem.par so that mfem.par can be linked with serial MFEM.
essentially this could be done by add #Ifdef MFEM_USE_MPI in the interface file in mfem/_par