Build Matrix for Serial and Parallel Version

- Assume one will build parallel tools only with parallel libSplash support
  (avoids blowing off the build matrix by a factor of two again)
- Exclude MPI example/ by default (same option -DPARALLEL=ON/OFF)
- include <mpi.h> only in examples itself
  (note: always include mpi.h BEFORE c-stdlibs due to conflicting
         defines in some c-libs)