Fix pthreads-win32 name in comments and documentation

* configure.ac (GC_WIN32_PTHREADS): Fix pthreads-win32 name.
* doc/README.macros (GC_WIN32_PTHREADS): Likewise.
* doc/README.win32: Likewise.
* include/gc.h (GC_WIN32_THREADS): Likewise.
* include/gc_config_macros.h (GC_WIN32_PTHREADS): Likewise.
* include/private/gc_locks.h (NUMERIC_THREAD_ID): Likewise.
* win32_threads.c (GC_pthread_join): Likewise.
* doc/README.win32: Add information how to build for Win32 with
pthreads-win32 using configure.
* include/private/gc_locks.h (NUMERIC_THREAD_ID): Adjust comment
(capitalize 1st word of a sentence, add dot at sentence end).
* win32_threads.c (GC_pthread_join): Join adjacent GC_WIN32_PTHREADS
checks; refine comment about pthreads-win32 id.