Fix configure for mingw-w64/32.

Fix configure for mingw-w64/32.

* configure.ac (PICFLAG, THREADDLLIBS): Define as empty for MinGW.
* configure.ac (have_pthreads): New definition (set to true unless
MinGW).
* configure.ac (HAVE_PTHREAD_H): New AM conditional (based on
have_pthreads value).
* tests/Makefile.am (test_atomic_pthreads_SOURCES,
test_atomic_pthreads_CPPFLAGS, test_atomic_pthreads_LDADD): Define
only if HAVE_PTHREAD_H.
* tests/Makefile.am (TESTS): Don't include test_atomic_pthreads unless
HAVE_PTHREAD_H.
* tests/Makefile.am (check_PROGRAMS): Define to TESTS value.