Turn on parallel marker by default for all multi-threaded builds

Turn on parallel marker by default for all multi-threaded builds

Note: if some target does not support PARALLEL_MARK, it should be
fixed (or a workaround should be added).

* CMakeLists.txt (enable_parallel_mark): Change default value to ON.
* CMakeLists.txt [enable_parallel_mark] (PARALLEL_MARK): Define only
along with GC_LINUX_THREADS, or GC_AIX_THREADS, or GC_HPUX_THREADS, or
GC_OPENBSD_THREADS, or GC_FREEBSD_THREADS, or GC_NETBSD_THREADS, or
GC_SOLARIS_THREADS, or GC_IRIX_THREADS, or GC_THREADS, or
GC_DARWIN_THREADS, or GC_OSF1_THREADS.
* configure.ac (PARALLEL_MARK): Always define unless threads are
disabled or enable_parallel_mark is "no".