libweston has separate version numbering from weston because of
development needs.

During development, weston version is major.minor.90 which will never be
a release version number. While developing, we may break the libweston
backward-compatibility, in which case libweston_major_version will be
bumped. This means that libweston_major_version > weston_major_version
but only during the development period and for the pre-releases. When
the official x.y.0 release is made, weston and libweston versions will
get synchronized as explained in releasing.txt.

The reason we do this is that e.g. during the weston 3.0.90 development
period we must be able to install libweston-4.so because the development
has broken the compatibility and so we cannot install it as libweston-3.so
anymore. However, we cannot bump weston to 4.0.90, because then the
official release would go backwards in numbers to 4.0.0.

This also means that weston pre-releases major.minor.9x may install
libweston-(major+1).so. There is also libweston-(major+1).pc file but it
will give the weston version as the version number. IOW, pkg-config
check for 'libweston-M < M.0.0' matches only the pre-releases of the
libweston major version M. Hence, 'libweston-M >= M.0.0' cannot be
satisfied by pre-releases.

The weston and libweston version numbers MUST be identical in all
releases except the pre-releases major.minor.9x.

When the 1.11.91 pre-release is made, the rules imply that libweston
version will be bumped from 0.0.0 to 1.11.91. The bumping will continue
up to the 1.12.0 release. After the bump to 1.12.90, the libweston
version may be bumped to 2.0.0. Then the rules imply that:
- 1.12.9x pre-releases install libweston 2.0.0
- the next .0 release is 2.0.0 containing libweston 2.0.0

If the 1.12 stable branch will see additional releases, those will be
numbered 1.12.1, 1.12.2, etc. with the libweston version being the same
as the release version number.

If we have release 2.0.91 without libweston major bump, then libweston
version will match the release version, leading up to 2.1.0.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>