move to saner SONAME

[cbouilla]

Currently, each release has its own SONAME so no two releases are binary compatible. However, the ABI doesn't change much and we can move to a more stable SONAME.

A user said: This is really import for Linux distributions. We are currently updating all the Sage dependencies in Debian [¹] and for M4RI/M4RIE it is especially important to have a sane SONAME, because other libraries (e.g. linbox) depend on it. So right now M4RI claims not only to break its own ABI with every update, but also the ABIs of the libraries depending on it (and they need a source upload with every M4RI update).