Support winpthreads

Support winpthreads

Winpthreads is a different pthread implementation for MinGW-w64.
This patch redefines GC_WIN32_PTHREADS to mean either pthreads-win32
or winpthreads.

* configure.ac (GC_WIN32_PTHREADS): Improve description to cover
"winpthreads" library.
* doc/README.macros (GC_WIN32_PTHREADS): Likewise.
* doc/README.win32: Likewise.
* include/gc_config_macros.h (GC_WIN32_THREADS): Mention "winpthreads"
library in comment.
* win32_threads.c (GC_pthread_join): Likewise.
* include/private/gc_locks.h (NUMERIC_THREAD_ID, THREAD_EQUAL,
NUMERIC_THREAD_ID_UNIQUE): Define to support winpthreads properly (if
GC_WIN32_PTHREADS).
* win32_threads.c (GC_PTHREAD_PTRVAL): Likewise.