Update build on Spider

Updated the build script for Spider. This was necessary because the system-wide Python on Spider (3.6) is too old.

• Rapthor now always uses a virtualenv, also when building.
• Problems with library and header file conflicts between system-wide HDF5 and our custom build version, when building with MPI support (module mpi), finally seem to be resolved. Both mpicc and some of the Schaap packages use -isystem to include HDF5 header files; this causes the wrong (system-wide) header files to be used during compilation. It looks like we can simply use gcc instead.