Add option to only build library (#4801)

Add option to only build library (#4801)

This PR would add an option to only build the CVC4 library and not the parser or executable. This can be used for projects that only intend to use CVC4 through the API.

It seems to be working now, but it's not necessarily the cleanest solution. In particular, if you'd like the polarity to be different I'm happy to change that. Polarity meaning something like "${WITH_BINARY}" STREQUAL "YES" instead of NOT "${LIB_ONLY} STREQUAL "YES" which is admittedly a little strange.