Alias of C_NULL_PTR to "NULL" clashes with Fortran intrinsic

[jtappin]

While I was cleaning up some code, I updated some of its pointer definitions to intialize the pointers using the intrinsic null() function.

e.g.
real, dimension(:), pointer :: ptr = null()

to ensure a valid result for any call to associated().

Unfortunately by providing the convenience alias of NULL for C_NULL_PTR, we have made the intrinsic Fortran function inaccessible in any module that USEs the gtk fortran modules.

Not sure what's the best to do with the conflict. I know I've used the NULL constant extensively in my code.

[vmagnin]

One solution would be to rename NULL in all the gtk-fortran files.
Another solution would be to locally rename NULL by using the following syntax:

use gtk, only: GTKNULL => NULL 

[jtappin]

Within the high-level routines, I've changed to the iso_c_binding native names (C_NULL_CHAR, C_NULL_PTR and C_NULL_FUNPTR).

[vmagnin]

I have replaced NULL, CNULL, FNULL by the iso_c_binding names in all the remaining files of the project (master and gtk3 branch).