Put variable/function declarations into extern 'C' in headers

Put variable/function declarations into extern 'C' in headers

Issue #201 (bdwgc).

Extern "C" should be only around declarations, not includes.
In particular, do not include 3rd-party headers (as well as our own
files as they could include others).

* include/gc.h [GC_PTHREADS]: Do not wrap include gc_pthread_redirects.h
into extern "C" block.
* include/gc_disclaim.h: Wrap variable and function declarations (but
not included headers).
* include/gc_inline.h: Likewise.
* include/gc_pthread_redirects.h [!GC_PTHREAD_REDIRECTS_ONLY]: Likewise.
* include/private/darwin_semaphore.h: Likewise.
* include/private/darwin_stop_world.h: Likewise.
* include/private/dbg_mlc.h: Likewise.
* include/private/gc_atomic_ops.h [GC_BUILTIN_ATOMIC]: Likewise.
* include/private/gc_hdrs.h: Likewise.
* include/private/gc_locks.h [THREADS]: 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 [GC_PTHREADS && !GC_WIN32_THREADS]:
Likewise.
* include/private/specific.h: Likewise.
* include/private/thread_local_alloc.h [THREAD_LOCAL_ALLOC]: Likewise.