Add COMPATIBILITY argument to ecm_setup_version().
Review Request #121696 - Created Dec. 27, 2014 and submitted
Lots of libraries will want to use SameMajorVersion to make sure
searching for version 1 of a library doesn't give you version 2, for
We may want to add another, custom compatibility mode for
KDE Frameworks-style versioning, where version x.90.z to x.99.z are
alpha/beta releases for version (x+1).
See unit tests - all pass.