Enable handle-fork by default

Enable handle-fork by default

Issue #174 (bdwgc).

* CMakeLists.txt (enable_handle_fork): New option (on by default).
* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT && enable_handle_fork]:
Define HANDLE_FORK macro for aix, cygwin, freebsd, haiku, hpux11,
irix, kfreebsd, linux, netbsd, openbsd, osf, solaris hosts.
* CMakeLists.txt [CMAKE_USE_PTHREADS_INIT && HOST=*-*-darwin*
&& enable_handle_fork && enable_parallel_mark]: Define HANDLE_FORK
macro; add comment.
* configure.ac (AC_ARG_ENABLE(handle-fork)): Allow "auto" and "manual"
values; update help message.
* configure.ac [$enable_handle_fork!=yes && $enable_handle_fork!=no
&& $enable_handle_fork!=manual && $THREADS=posix]: Define HANDLE_FORK
macro for aix, cygwin, freebsd, haiku, hpux11, irix, kfreebsd, linux,
netbsd, openbsd, osf, solaris hosts (and also for darwin if
enable_parallel_mark); add comment.