New macro (NO_MSGBOX_ON_ERROR) to avoid message box on GC abort (Win32)

New macro (NO_MSGBOX_ON_ERROR) to avoid message box on GC abort (Win32)

NO_MSGBOX_ON_ERROR macro disables Win32 MessageBox() invocation.

* doc/README.macros (NO_MSGBOX_ON_ERROR): Document.
* include/private/gcconfig.h [!MSGBOX_ON_ERROR && !NO_MSGBOX_ON_ERROR
&& !SMALL_CONFIG && MSWIN32 && !MSWINRT_FLAVOR && !MSWIN_XBOX1]
(MSGBOX_ON_ERROR): Define new macro.
* misc.c (GC_win32_MessageBoxA): Define only if MSGBOX_ON_ERROR.
* misc.c [!PCR && !SMALL_CONFIG] (GC_default_on_abort): Do not call
GC_win32_MessageBoxA() unless MSGBOX_ON_ERROR is defined.