Define GC_THREADS instead of GC_x_THREADS in Makefiles

Define GC_THREADS instead of GC_x_THREADS in Makefiles
(code refactoring)

* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT] (SRC): Add
thread_local_alloc.c in a single place.
* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT]: Define GC_THREADS macro
instead of GC_LINUX_THREADS, GC_AIX_THREADS, GC_HPUX_THREADS,
GC_OPENBSD_THREADS, GC_FREEBSD_THREADS, GC_NETBSD_THREADS,
GC_SOLARIS_THREADS, GC_IRIX_THREADS, GC_DARWIN_THREADS, GC_OSF1_THREADS.
* configure.ac [THREADS=posix]: Likewise.
* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT]: Handle
*-*-aix*, *-*-*freebsd* (including kfreebsd), *-*-gnu*, *-*-irix*,
*-*-openbsd*, *-*-osf*, *-*-solaris* targets in the same ways as
*-*-*linux*.
* Makefile.direct (CFLAGS): Do not mention GC_HPUX_THREADS in comment.
* README.QUICK: Refine documentation about macro and configure option
for builds with (and without) threads support.
* configure.ac (GC_AIX_THREADS, GC_DARWIN_THREADS, GC_FREEBSD_THREADS,
GC_HPUX_THREADS, GC_IRIX_THREADS, GC_LINUX_THREADS, GC_NETBSD_THREADS,
GC_OPENBSD_THREADS, GC_OSF1_THREADS, GC_SOLARIS_THREADS,
GC_WIN32_THREADS, GC_RTEMS_PTHREADS): Remove AH_TEMPLATE.
* configure.ac [THREADS=posix]: Handle *-*-aix*, *-*-irix* targets in
the same ways as *-*-*linux*.
* configure.ac [THREADS=win32 || THREADS=dgux386 || THREADS=aix
|| THREADS=rtems]: Define GC_THREADS macro instead of GC_WIN32_THREADS,
GC_DGUX386_THREADS, GC_AIX_THREADS, GC_RTEMS_PTHREADS.
* doc/README.darwin: Replace GC_MACOSX_THREADS with GC_THREADS.
* doc/README.environment (doc/README.environment): Replace
GC_OSF1_THREADS with OSF1.
* doc/README.hp: Replace GC_HPUX_THREADS with GC_THREADS.
* doc/README.linux: Replace GC_LINUX_THREADS with GC_THREADS.
* doc/scale.html: Likewise.
* doc/README.macros (GC_SOLARIS_THREADS, GC_IRIX_THREADS,
GC_HPUX_THREADS, GC_LINUX_THREADS, GC_OSF1_THREADS, GC_FREEBSD_THREADS,
GC_NETBSD_THREADS, GC_OPENBSD_THREADS, GC_DARWIN_THREADS,
GC_AIX_THREADS, GC_DGUX386_THREADS, GC_WIN32_THREADS): Add note that
the macro is deprecated (GC_THREADS one should be used instead).
* doc/README.sgi: Replace GC_IRIX_THREADS with GC_THREADS.
* doc/README.solaris2: Do not mention GC_SOLARIS_THREADS.
* doc/gcinterface.html: Do not mention GC_XXXX_THREADS.