[patch,libgfortran] Fix configure test for weakref support

[FX]

This patch fixes PR 47571 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47571) 
by providing an adequate configure check (with AC_LINK_IFELSE) for weakref 
(which we use in intrinsics/system_clock.c).

Tested by building libgfortran on x86_64-pc-linux-gnu, where SUPPORTS_WEAKREF 
gets defined to 1 as it should, and x86_64-apple-darwin14, where 
SUPPORTS_WEAKREF gets defined to 0 as it should. Then regtested on both targets.

OK to commit to trunk?

FX


2015-08-14  Francois-Xavier Coudert  <fxcoud...@gcc.gnu.org>

        PR libfortran/47571
        * acinclude.m4 (LIBGFOR_GTHREAD_WEAK): Remove.
        (LIBGFOR_CHECK_WEAKREF): New test.
        * configure.ac: Call LIBGFOR_CHECK_WEAKREF instead of
        LIBGFOR_GTHREAD_WEAK.
        * config.h.in: Regenerate.
        * configure: Regenerate.
        * intrinsics/system_clock.c: Use SUPPORTS_WEAKREF instead of
        SUPPORTS_WEAK and GTHREAD_USE_WEAK.

weakref.diff
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