binding: add "-undefined dynamic_lookup" flag on OS X
binding: add "-undefined dynamic_lookup" flag on OS X

Needed for node-gyp v2.0.0

rc0x03/libxmljs,rc0x03/libxmljs,Visya/libxmljs,Konsus/libxml,libxmljs/libxmljs,improvisio-software/libxmljs,sergeche/libxmljs,sergeche/libxmljs,libxmljs/libxmljs,polotek/libxmljs,polotek/libxmljs,hildjj/libxmljs,polotek/libxmljs,hildjj/libxmljs,rc0x03/libxmljs,rchipka/libxmljs,gagern/libxmljs,rchipka/libxmljs,improvisio-software/libxmljs,rchipka/libxmljs,gagern/libxmljs,libxmljs/libxmljs,Konsus/libxml,Visya/libxmljs,hildjj/libxmljs,sergeche/libxmljs,Konsus/libxml,improvisio-software/libxmljs,gagern/libxmljs,Visya/libxmljs