You need pkgconfig installed to compile this, even if using the --enable-gtk1 option. This plugin requires the 1.6 version of mozilla or gecko-sdk in order to work correctly with the Opera Browser. The easy and smaller way is to use the gecko-sdk-1.6. It doesn't need to be compiled. Just unpack it and give the path in the configure options. You can also give the opera directory as the mozilla-five-home so that the plugins get installed to the opera directory instead of to your firefox, mozilla or seamonkey path. This plugin seems to work fine for use with any firefox, mozilla or seamonkey browser version.