Make dependencies to libthai, cairo and xft optional.

This branch adds meson features for disabling the dependencies libthai, cairo and xft such that they won't be used even if present on the system. Changed meson dependencies for some tests accordingly.

This allows you to compile a pango library that works on systems where said dependencies are not present even though they are present on the build system. This also avoids the need to provide static libraries for the dependencies, if creating a static pango library, and not actually needing these deps.