re PR boehm-gc/48514 ([4.6] boehm gc incorrectly compile using __declspec(dllexport) on i686-w64-mingw32 target)

        PR boehm-gc/48514
        * include/gc_config_macros.h (GC_DLL): Define it for mingw-targets
        only, if we are actual in boehm-gc's build and DLL_EXPORT
        is defined.

From-SVN: r184100