* configure.ac: Introduce extra_ldflags_libgc. Use it for Darwin.
* configure: Regenerate.
* Makefile.am (libgc_la_LDFLAGS): Use extra_ldflags_libgc.
* Makefile.in: Regenerate.
* include/Makefile.in: Regenerate.
* include/private/gcconfig.h: Enable MPROTECT_VDB for all Darwin
targets. Remove comments. Prepare ppc64 support for Darwin.