Install versioned library

[eaaltonen]

Note

The library maintainers should review and consider when the library ABI changes. Substitute appropriate value for SOVERSION

Commit

• Set library VERSION and SOVERSION

With soversion and version specified, install target will install the
library with the specified version and also create the proper symlink.

Change

make install before:

libdocopt.so

make install after:

libdocopt.so -> libdocopt.so.0
libdocopt.so.0 -> libdocopt.so.0.6.2
libdocopt.so.0.6.2

Fixes #111.

[eaaltonen]

This has already been requested in PR#66, it seems.

[jaredgrubb]

Yes, this is a dup, but I think you have a slightly more correct approach. I'm trying to fix the CI stuff and then we can merge.

[jaredgrubb]

Worked well enough so just going to merge in.