What's the GEMPAK Fortran 90 interface?

For those who haven't snickered at anybody recently, here's one for you:

I see that if I stay with the 'FC = pgf77' in
$NAWIPS/config/Makeinc.linux.pgi, the make log reports that the Fortran
90 interface won't be built -- as it says if I use the default f77
(g77). If I change this to pgf90, I saw some compile errors, which I
didn't keep around.

So, what's the Fortran 90 interface and what is its significance?

Thanks,
-Neil
--
Neil R. Smith, Comp. Sys. Mngr.         neils@xxxxxxxx
Dept. Atmospheric Sci., Texas A&M Univ. 979/845-6272 FAX:979/862-4466