[gfortran,patch] Don't rewrite module files when they haven't changed

[FX Coudert <fxcoudert at gmail dot com>]

:ADDPATCH fortran:

[Janis: this patch contains a testsuite new procedure to scan  
modules, for which I need your approval. See the last paragraph of  
the mail.]

Building large Fortran codebases efficiently requires dependency  
tracking. When using gfortran, this is currently hampered by the fact  
that, given a file code.f90 that contains a module use_me, when  
code.f90 is recompiled after having changed, the module file  
use_me.mod is overwritten on the disk, even if its content hasn't  
changed.

This patch solves this by adding, on the second line of module files,  
a MD5 sum of the contents of the module file itself (except the first  
two lines). Furthermore, when a module is created, it's written to  
disk in a use_me.mod0 file, which is then only renamed into  
use_me.mod if it is different from the old one (as determined by  
reading the old MD5 sum and comparing). Thus, we don't update module  
files that haven't changed.

The patch comes with a testcase, and a new dejagnu procedure, scan- 
module, to scan module files. Janis, I need your approval for that  
one, and will welcome comments!

This was regtested on x86_64-linux, and I will test on i386-pc- 
mingw32 before committing. OK for mainline?

FX