configure: Test linking pthreads before using it

This avoids enabling pthreads if only pthreads-w32 is available.
pthreads-w32 provides pthread.h but has a link library with a
different name (libpthreadGC2.a).

Generally, always using win32 threads when on windows would be
sensible.

However, libstdc++ can be configured to use pthreads (winpthreads), and
in these cases, standard C++ headers can pollute the namespace with
pthreads declarations, which break the win32 threads headers that
declare similar symbols - leading us to prefer pthreads on windows
whenever available (see d167a1ae and bug 1132).

Change-Id: Icd668ccdaf3aeabb7fa4e713e040ef3d67546f00

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