[gfortran] Fix gfortran include problem

[FX Coudert]

Attached patch fixes PR fortran/20811, where gfortran is not able to 
include files located in the same directory as the source file, if it's 
not the current working directory (In all this mail, the inclusion I'm 
speaking of is the fortran keyword "include"). This is a regression 
w.r.t. g77, and all commercial compilers. Moreover, gfortan does by 
default include files from the current working directory (cwd), while 
g77 doesn't.

The patch fixes both erroneous behaviours. It is tested and regtested 
on ppc-darwin. OK for mainline? OK for 4.0?

:ADDPATCH fortran:

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