CMakeLists.txt: default to shared libraries by default

[trofi]

raptor2 provides a library and depends on a few other libraries on it's own (like libxml2).

Some downstream projects are not very careful to include all the needed libraries from .pc files and use just raptor2 one.

This sometimes causes linkage failures like guitarix: https://hydra.nixos.org/log/4hznma7pl7wvjdhm3x2lr84zpj07s2ax-guitarix-0.42.1.drv In this case linkage failure happens via underlinked liblrdf.so.

While it's a bug in liblrdf.so I think it would be reasonable to enable shared libraries by default for use in distributions as is.

This change enables shared library by default.