Do type-punning via union (instead of pointer type cast) to enable safe
'-fstrict-aliasing' compiler optimization option.
-Support HANDLE_FORK if USE_WINALLOC for Cygwin.
+Support CAN_HANDLE_FORK if USE_WINALLOC for Cygwin.
-Properly implement HANDLE_FORK if GC_dirty_maintained for Darwin (both single
-and multi-threaded modes).
+Enable GC_set_handle_fork(1) for Darwin with GC_dirty_maintained on (both
+single and multi-threaded modes).
== FIXME tasks ==