Allow incremental GC on Cygwin

Allow incremental GC on Cygwin

Both GWW- and mprotect-based implementations should work. The first one
works only if the collector is compiled with -D USE_WINALLOC passed to
CFLAGS.

* include/private/gcconfig.h [(I386 || X86_64) && CYGWIN32]
(MPROTECT_VDB): Define.
* include/private/gcconfig.h [(I386 || X86_64) && CYGWIN32
&& USE_WINALLOC] (GWW_VDB): Define.
* include/private/gcconfig.h [(I386 || X86_64) && CYGWIN32 && USE_MMAP]
(NEED_FIND_LIMIT, USE_MMAP_ANON): Do not define if USE_WINALLOC.
* include/private/gcconfig.h [GWW_VDB && !USE_WINALLOC && !CPPCHECK]:
Produce an invalid config error.