GitLab

* cmake: Use major.minor for the SONAME for C++ and C#

This helps to localize the ABI changes between versions and makes
possible to install different versions of GRPC side-by-side on the same
root filesystem. This is still not an ideal solution, but at least the
expectation would be whenever the core version has changed, the SONAME
would be different for any other related artifacts.
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

* doc: Add section about ABI compat to BUILDING.md
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>