[Patch, fortran] PR20896 - dummy procedures interfaces

[Paul Thomas]

Dominique

By the way, I think that those compilers that have the original PR 
testcase as ambiguous have it wrong.  Testing the characteristics of 
dummy procedures with explicit interfaces, as required by the standard, 
sort this out.

The only other reading that I see of the standard is that external 
should only be matched with external.

Paul