build: Include the Wayland minor version in libraries' ABI versions

The ABI of a shared library on Linux is given by a major version, which
is part of the SONAME and is incremented (rarely) on incompatible
changes, and a minor version, which is part of the basename of the
regular file to which the SONAME provides a symlink.

Until now, the ABI minor version was hard-coded, which means we can't
tell which of a pair of Wayland libraries is newer (and therefore
likely to have more symbols and/or fewer bugs).

libwayland-egl already had ABI major version 1, so we can use the
"marketing" version number as the ABI major.minor version number
directly, so Wayland 1.19.90 would produce
libwayland-egl.so.1 -> libwayland-egl.so.1.19.90.

libwayland-cursor and libwayland-server have ABI major version 0,
and OS distributions don't like it when there's a SONAME bump for no
good reason, so use their existing ABI major version together with
the "marketing" minor version:
libwayland-cursor.so.0 -> libwayland-cursor.so.0.19.90.

If the Wayland major version number is incremented to 2, we'll have to
rethink this, so add some error() to break the build if/when that
happens. Assuming that Wayland 2.0 would involve breaking changes,
the best way would probably to bump all the SONAMEs to
libwayland-foo.so.2.

Resolves: https://gitlab.freedesktop.org/wayland/wayland/-/issues/175


Signed-off-by: Simon McVittie <smcv@collabora.com>