This is a big rework of how dependencies are handled internally. All the
calls to CMake functions changing directory-wide properties that were
previously used to link to external and internal dependencies have been
replaced with their "target_" counterparts. This makes it possible to
express more clearly the relationships between different targets and
should also make the build script more robust in general.
In doing this, I've also made it possible to link to system libraries if
so desired; the versioned calls to find_package() will make sure that
the found dependencies are compatible with the ones required by Cemu,
and will abort the build otherwise.
Cemu's internal targets are deeply interconnected, making it hard to
fully benefit from a target-based approach. Nonetheless, I did my best
to mark PUBLIC dependencies as such, for example when an internal target
like CemuGui exposes a dependency as part of its API, i.e. it includes
a third party header in one of its public headers.
This will significantly help with improving the build experience on
Linux, thus helping a bit with the resolution of #1.
