GitLab

- also add option to disable "null" module for now

- works for both mpi and non-mpi builds
- unlike darshan_core_time(), it does not compute relative timestamp and
  thus doesn't require darshan core to be initialized

- add new --without-mpi to bypass all mpicc checks
- add --disable-xxx-mod to individually enable/disable each module
- add report of enabled/disabled modules at end of configure
- does NOT report compiler version yet
- does NOT report whether MPI is enabled or not yet

for MPIIO, PNETCDF, and HDF5 modules.