Change some optional dependencies from being optional to being mandatory
or being mandatory but behind on-by-default cmake flags. Eg: instead of
only compiling Wayland support if we find the appropriate libraries, we
always require the libraries unless the user specifies WITH_WAYLAND=OFF.

Optional libraries are hard to discover by packagers (since they don't
see an error once we add them) and create lots of possible build flavors
with a different features that can confuse users.