Remove GC_GNU_THREADS macro (HURD)

Remove GC_GNU_THREADS macro (HURD)
(code refactoring)

* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT && *-*-gnu*] (GC_GNU_THREADS):
Define GC_THREADS macro instead.
* configure.ac [THREADS=posix && *-*-gnu*] (GC_GNU_THREADS): Likewise.
* configure.ac (GC_GNU_THREADS): Remove AH_TEMPLATE.
* include/gc_config_macros.h: Remove check for GC_GNU_THREADS.
* include/private/gcconfig.h [!CPPCHECK]: Remove consistency check for
GC_GNU_THREADS.
* pthread_support.c [GC_PTHREADS && !GC_WIN32_THREADS] (GC_get_nprocs):
Replace GC_GNU_THREADS to HURD macro check.
* tools/threadlibs.c (main): Replace GC_GNU_THREADS to HURD and
GC_THREADS macros check.