[GTK] Enable the Wayland target if GTK+ dependency is found

https://bugs.webkit.org/show_bug.cgi?id=121704

Reviewed by Gustavo Noronha Silva.

The Wayland target should be enabled by default if the GTK+ dependency is of version 3.9.14 or later.

* Source/autotools/FindDependencies.m4: If the Wayland target was not strictly disabled, the GTK+ dependency
is tested if it can provide the Wayland GDK backend, and that the version of that backend matches the version
of the master GTK+ dependency for which we already tested. If found, the target is enabled, otherwise we either
warn or error out, depending on whether the build target was set to auto or completely disabled through configuration.
* Source/autotools/ReadCommandLineArguments.m4: Switch the default Wayland target status to 'auto', meaning
it will be disabled if the GTK+ dependency is not found.
* Source/autotools/Versions.m4: Require GTK+ 3.9.14 for the Wayland target only.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@156328 268f45cc-cd09-0410-ab3c-d52691b4dbfc