Require Meson 0.54 for Gtk+3
When configuring the project with Meson I get all these warnings:
WARNING: Project specifies a minimum meson_version '>= 0.48.0' but uses features which were added in newer versions:
* 0.49.0: {'/ with string arguments'}
* 0.51.0: {'dep.get_variable'}
* 0.53.0: {'Fallback without variable name'}
* 0.54.0: {'internal arg in dep.get_variable'}
Fixes #3927 (closed) (which was closed before anything was actually done)