Enable gc.h inclusion by client without implicit include windows.h (Win32)

Enable gc.h inclusion by client without implicit include windows.h (Win32)

* doc/README.macros (GC_DONT_INCLUDE_WINDOWS_H): Document.
* include/gc.h: Include windows.h unless GC_DONT_INCLUDE_WINDOWS_H
(only for Win32).
* include/gc.h (DECLSPEC_NORETURN): Define to __declspec(noreturn) by
default if windows.h not included (instead of empty).
* include/gc.h (GC_WIN32_SIZE_T): Define to DWORD (or unsigned long)
unless _WIN64 (instead of GC_uintptr_t which is unsigned int for
32-bit target).
* include/gc.h (GC_DllMain, GC_CreateThread, GC_ExitThread): If WINAPI
undefined (i.e., if windows.h not included by or prior to gc.h) then
declare the prototype using built-in C types instead of Windows types.