build: Improve dependency search
Hi,
This updates the build files in the following ways:
- Require Meson 0.60 or later, so that:
- We can search for libxml2 via pkg-config and CMake in one single go, before using libxml's fallback build.
- Include Vulkan in the dependencies if it is found in ways other than pkg-config, so that builds won't break if GTK4 is built with Vulkan support.
With blessings, thank you!