Improvements to meson build system

This is a set of improvements for meson build systems. I would like to highlight that documentation generation with valadoc is working now, so meson support is on par with autotools now.

Regarding valadoc support, my idea was to add support for it in meson (I mean the build system), but due to my lack of time for doing so, and due to the fact that gitg is the only program I know that is using it, I have decided to implement it as a custom target.