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

[François-Xavier Coudert]

> Well, I don't know autoconf/automake enough to judge the patch itself,
> but I can test on my own i686-linux to i386-mingw32 cross. This is
> under way, I'll report when it's done.

Hum... I first built a build=host=i686-linux, target=i386-mingw32
compiler without the patch, and it works fine. Wasn't it supposed to
fail in some way?

FX