build: Various clean-ups
Various clean-ups to make life of downstreams easier. In particular, on NixOS, we like to install installed tests into a different prefix so we patch Meson to get rid of the irrational single prefix limitation and then add extra options to projects for specifying the installed tests prefix.
Downstream changes: https://github.com/NixOS/nixpkgs/commit/177a4f6c91abb6aad5df0cb024584aec627f7829
Edited by Jan Tojnar