webkitgtk: update to version 2.18.0

Release notes:
    https://webkitgtk.org/2017/09/11/webkitgtk2.18.0-released.html



No corresponding WebKit Security Advisory (WSA) has been published.

All patches have been applied upstream.

This also bumps the required target GCC version, due to the WebKit code
now using more modern C++ features which were introduced in version
5.x of the compiler.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
[Arnout:
 - propagate dependency to midori;
 - mention in commit message why patches were removed.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>