Replace conditional extern 'C' blocks begin/end with EXTERN_C_BEGIN/END

Replace conditional extern 'C' blocks begin/end with EXTERN_C_BEGIN/END
(code refactoring)

Issue #206 (bdwgc).

Note: EXTERN_C_BEGIN and EXTERN_C_END are not available in the GC
public headers.

* include/private/darwin_stop_world.h: Replace the conditional
extern "C" block begin/end with EXTERN_C_BEGIN and EXTERN_C_END,
respectively.
* include/private/dbg_mlc.h: Likewise.
* include/private/gc_hdrs.h: Likewise.
* include/private/gc_locks.h: Likewise.
* include/private/gc_pmark.h: Likewise.
* include/private/gc_priv.h: Likewise.
* include/private/gcconfig.h: Likewise.
* include/private/pthread_stop_world.h: Likewise.
* include/private/pthread_support.h: Likewise.
* include/private/specific.h: Likewise.
* include/private/thread_local_alloc.h: Likewise.
* os_dep.c [MPROTECT_VDB && DARWIN]: Likewise.
* tests/test.c [GC_AMIGA_FASTALLOC && AMIGA]: Likewise.
* include/private/gcconfig.h (EXTERN_C_BEGIN, EXTERN_C_END): Define
macro.