FindMPI: Fix flags handling.
Hi, I've noticed to further issues with FindMPI:
- Leading spaces of compiler options, i.e. -fxxxx aren't being stripped. This breaks FindMPI on Cygwin for me, as mpif90 passes -fexceptions for some reason.
- the includes array isn't being reset to an empty variable before use but the rest of the code expects it to be empty until a suitable path was found.