meson.build: check gtk targets before building
We have two different dependencies on Wayland: GTK support and the wayland-protocols we use directly. If we have GTK support but wayland-protocols is not installed at meson configure time, our build fails.
To avoid having multiple ifdefs in the code, let's define two new ones: HAVE_GTK_WAYLAND and HAVE_GTK_X11, both set if GTK supports that particular target (from pkgconfig) and we have the other support libraries we need.