[PATCH] Fix thread-safety follow-out for cross builds (PR libfortran/24991)

[Paolo Bonzini]

> I doubt many/any of the main gfortran hackers have much experience
> with cross building gfortran.   To get this patch reviewed and
> committed in short order, you'll probably need to ask someone with
> much more GCC infrastructure knowledge than the gfortran hackers.

The patch seems fine, but I didn't look at it *so* thoroughly.  Note
that it is not triggered by a cross build but rather by a build with
"./configure && make" (srcdir == builddir).