Implementation of F2018:18.4 C descriptors and ISO_Fortran_binding.h

[Thomas Koenig]

Hi Paul,

I think your patch looks good, in principle - I will send
you a clean patch against a reasonably recent tree in a
separate mail.

I think the description of the new procedures can come later.

There is just one point: I think the ISO_Fortran_binding.h needs to
go where the standard C headers go, and I have no real (or integer)
clue how this can be accomplished.

Ideas, anybody?

Regards

	Thomas