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

[François-Xavier Coudert]

> I doubt many/any of the main gfortran hackers have much experience
> with cross building gfortran.

Oh, I certainly think that Paul Brook has quite a lot of experience in
building cross-compilers :) But we don't "see" him much these days...

> BTW, I did not see anything in the patch that set off alarms, but
> then again I don't know to go about testing the patch.

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.

By the way, Jakub, you didn't specify on which platforms you did test
it (be them native or cross).

FX