Add webkit2-gtk3 2.6.4.

This port only supplies the WebKit2 library for gtk3, and has some API changes.
The webkitgtk project switch to cmake. Due to issues I don't understand

yet we can't use USES=cmake yet.

Use USES=compiler:c++11-lib to replace custom dancing around with clang

and OSVERSION.

Make 8.x as IGNORE, because of c++ and other issues.