make plain nc dump style a compile time option as well

according to the package author, lars pastewka, some machines
may have only parallel netcdf and not the serial netcdf installed.
hence both netcdf dump styles neet to be optional depending on
the compile time setting in lib/netcdf/Makefile.lammps
The makefile has been set up to auto-detect the necessary flags
for most regular and correct installations.