[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

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