Making Cairo backend mandatory for building?

[Jean Abou Samra]

Even though the Cairo backend is advertised as experimental, it is
starting to get some use; see for example the message today on -user
about a new OOoLilyPond release supporting it. This makes me wonder:
now that the official binaries have Cairo, isn't it time to require
it for the build and ditch --enable-cairo-backend so that everyone has
it, including those who use distro packages? To be clear, this wouldn't
be for 2.24, which is in build freeze mode, but we could do it in master
after branching. We're going to do it sooner or later in the 2.25 series
anyway, as we eventually want Cairo to become the default.