When building shared library, SONAME is not set.
SONAME is useful to track ABI compatibility changes, so that programs can pick up the correct version of a library. The SONAME should be updated every time that a library changes its ABI (e.g., symbol deletion, behavior change, ...), so that it is possible to determine if a program should be recompiled when a library is updated.

Attached you can find a patch that adds SONAME. You should update the SONAME every time you release a version of the library that is not backward compatible at the ABI level. SONAME is completely unrelated to release version and mostly unrelated to API changes.