Link libstdc++ statically (avoid dependency)

Please merge after testing. Test builds can be found at einstein2:/tmp/eah_gfx_refresh_win64_static.zip

src/starsphere/Makefile.mingw

@@ -35,6 +35,7 @@ LIBS += -lboinc_graphics2 -lboinc_api -lboinc
 LIBS += -Wl,-Bdynamic
 LIBS += $(shell $(STARSPHERE_INSTALL)/bin/sdl2-config --static-libs)
 LIBS += -lopengl32 -lglu32
+LIBS += $(shell $(CXX) -print-file-name=libstdc++.a)
 
 LDFLAGS += -static-libgcc