This change emulates Libtool to make sure Libtool and CMake agree on the ABI version.
To test, one needs to simulate a release with backward-compatible API changes, which means the following changes in configure.ac and CMakeLists.txt:
- incrementing of 
*_LIB_VERSION_CURRENT - setting 
*_LIB_VERSION_REVISIONto zero - incrementing of 
*_LIB_VERSION_AGE