A pkgconfig file is already provided, which enables using `thorvg` once
it is installed. However, this file is not, and cannot be, available at
setup time if using `thorvg` as a subproject.
In such cases, Meson provides the `override_dependency` mechanism for a
subproject to tell its parent how to use it.
Introduce project skeleton such as build envrionment and interfaces
to build up code further cooperatively.
Change-Id: Ie9ac38991f609d33637283134906d7cc3f2ac19e
