[libgfortran,patch] Add compiler check when configuring libgfortran

[FX Coudert]

Hi gfortranners,

As this patch was designed with help from build machinery  
specialists, and it doesn't really involve any change to the library  
itself, I will commit it in a few days if noone complains. I will  
then wait a two weeks before backporting it to 4.2, unless someone  
raises a concern with that backport, of course.

Regards,
FX


> Attached patch fixes a long-standing bug in libgfortran configury by
> adding an explicit check that we have a working gfortran compiler, and
> otherwise error out. Without this patch, a miscompiled (or mislinked)
> gfortran will only induce an error late in the libgfortran build, and
> with a very cryptic error message (see PR 26893 for details). With
> this patch, a non usable compiler will yield the following error
> message:
>
> checking whether the GNU Fortran compiler is working... no
> configure: error: GNU Fortran is not working; please report a bug in
> http://gcc.gnu.org/bugzilla, attaching
> /home/fxcoudert/svn/debug3/ibin/i686-pc-linux-gnu/libgfortran/ 
> config.log
>
>
> Bootstrapped and tested on i686-linux. OK for mainline? Would it be OK
> to backport this to 4.2, a few weeks after it has been commited to
> mainline? (I think it's an annoying bug for users compiling on unusual
> platforms or using unusual configurations).