build: use separate versioning schemes for API & ABI

build: use separate versioning schemes for API & ABI

Commit 783ff2e3c707 ("API: start certain enums from 1") broke the ABI
compatibility (while keeping the source compatibility). We need to
indicate that to the users by changing the SONAME string. Up until now
the SONAME would correspond with the major release number. We now need
to have a separate versioning schemes for API and ABI.

Keep using the same MAJOR.MINOR.RELEASE scheme for the API, but switch
to using the libtool versioning - CURRENT.REVISION.AGE - for ABI. Start
the ABI version from 2.0.0 to indicate that it changed since 1.0.0.

Define a separate ABI version for the C++ bindings, but start with 0.0.0
since it was not a part of any release yet.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>