gyp: Remove useless pkg-config variable definitions
gyp: Move wayland.gyp to wayland/ directory

wayland.gyp is the configuration file responsible for the libwayland and its
toolkit implementation; it naturally belongs to wayland/ therefore.
gyp: Specify Wayland and Mesa version

The versions set in there are figurative and are the ones that just work in my
machine. We may want to adjust them and the APIs accordingly whenever a new
version of those libraries are out and stable.