CMake MPI fixes
This patch fixes several things around Gmsh's CMake MPI:
- Forcing the compiler is deprecated https://cmake.org/cmake/help/v3.6/module/CMakeForceCompiler.html
-
MPI_INCLUDE_DIR
never did anything;MPI_{C,CXX}_INCLUDE_PATH
is the correct variable -
MPI_LIBRARIES
is deprecated; useMPI_{C,CXX}_LIBRARIES
instead