[PROPOSAL] Require Fortran 95

[derobins]

PROBLEM

The HDF4 Fortran wrappers are fundamentally based on Fortran 77. This has at least two problems:

• The integer type is limited to 32-bits, which is a problem since some API calls try to jam pointers into Fortran integers
• The Autotools build system contains a lot of Fortran 77 stuff and I'm afraid to touch it

PROPOSED FIX

• Bring over the Fortran build system improvements from HDF5
• Require Fortran 95+
• See if we can update the HDF4 Fortran integer types to make the unsafe Fortran calls safe again (even if jamming pointers into integers for posterity is fundamentally unwise)

HOW WOULD USERS BE AFFECTED?

PRO
➕ HDF4 would be more usable in modern Fortran environments
➕ Building HDF4 + Fortran would probably be easier
➕ It would be easier to bring Fortran improvements over from HDF5
➕ Autotools build hacks and warnings would go away
➕ Maybe we can make the Fortran wrappers safer

CON
➖ Anyone using Fortran 77 will no longer be able to use the Fortran wrappers
➖Some aspects of building the Fortran wrappers may change

[derobins]

This will be addressed in 4.4.0 (#424).