Failure building with mpicc in current master

[WardF]

Appears to be an issue with EXTERNL not being defined properly in netcdf_par.h. Fixing now, should be pretty straight-forward.

[gsjaardema]

I thought that the correct usage was to require that netcdf.h be included prior to including netcdf_par.h wherever it was used. See #1502

[WardF]

You're right, of course, @gsjaardema. Reworking so that the fix is in line with correct usage.