Compiling when libnetcdf.a and libnetcdff.a are in different folders

On our system we install netCDF-c and netCDF-fortran to different folders so we can manage different versions of each separately. The Makefile and make_fire_emis script does not support this as there is only one NETCDF_DIR variable. Please add support for a second such variable. I have a code modification for this which I will propose via pull request shortly.