meson: check for graphics before running gtk-builder-tool

gtk-builder-tool is used to valide ui files. Just checking that the tool
is installed before using it is not enough: we must also check whether
we have graphics.

This is needed for builds in Fedora container, where:
- the package gtk3-devel pulls in gtk-builder-tool
- it's a container, there's no graphics

Thanks to the meson community:
https://github.com/mesonbuild/meson/issues/9