Get the versioning right

[alexxcons]

Currently the shared library is named libgnuradio-digitizers-5.1.2.0.so.0.0.0 and there are links to it generated, e.g:

libgnuradio-digitizers-5.1.2.0.so -> libgnuradio-digitizers-5.1.2.0.so.0.0.0
.
That is not very helpful if one wants to link just against the API version 5 and does not care for the minor/tiny versions.

It should rather look like this:
libgnuradio-digitizers.so.5.1.2
libgnuradio-digitizers.so.5.1 -> libgnuradio-digitizers.so.5.1.2
libgnuradio-digitizers.so.5 -> libgnuradio-digitizers.so.5.1.2
libgnuradio-digitizers.so -> libgnuradio-digitizers.so.5.1.2
or
libgnuradio-digitizers-5.so.1.2
libgnuradio-digitizers-5.so.1 -> libgnuradio-digitizers-5.so.1.2
libgnuradio-digitizers-5.so -> libgnuradio-digitizers-5.so.1.2

Just spend half an hour trying to figure out that miracles of cmake .. learned there are VERSION and a SOVERSION variables which theoretically should do the job .... though setting them seems to have no effect.