Add something like:
DShowVideoDecoderPlugin.dll: c:/ohshima/vc/dshowlib/dshowlib/Release/SampleGrabber.lib
$(MD) -p -v Release\\DShowVideoDecoderPlugin
$(MAKE) -C ../DShowVideoDecoderPlugin -f ../win32/Makefile.mingw32 DLLDIR=DShowVideoDecoderPlugin DLLNAME=DShowVideoDecoderPlugin.dll DLL XDEFS=-DSTANDALONE
$(DLLTOOL) --output-def $(OUTDIR)/DShowVideoDecoderPlugin/DShowVideoDecoderPlugin.def --output-exp $(OUTDIR)/DShowVideoDecoderPlugin/DShowVideoDecoderPlugin.exp --output-lib $(OUTDIR)/DShowVideoDecoderPlugin/DShowVideoDecoderPlugin.lib $(OUTDIR)/DShowVideoDecoderPlugin/*.o c:/ohshima/vc/dshowlib/dshowlib/Release/SampleGrabber.lib
$(DLLWRAP) -mwindows -def $(OUTDIR)/DShowVideoDecoderPlugin/DShowVideoDecoderPlugin.def -o $(OUTDIR)/DShowVideoDecoderPlugin.dll $(OUTDIR)/DShowVideoDecoderPlugin/*.o c:/ohshima/vc/dshowlib/dshowlib/Release/SampleGrabber.lib $(LIBS) $(OUTDIR)/DShowVideoDecoderPlugin/DShowVideoDecoderPlugin.exp
strip --strip-all $(OUTDIR)/DShowVideoDecoderPlugin.dll
to Makefile.mingw32 (hmm, this can be much cleaner...).