Allow CMake to build both shared and static libraries

[brad0]

The autotools infrastructure for libmatroska and libebml allowed for building both shared and static libraries.

The new CMake infrastructure appears to only be able to build shared OR static libraries depending on the setting of BUILD_SHARED_LIBS instead of enabling both.

[mbunkus]

I'm not going to spend any time on this as I don't know a lot about cmake. That being said, I'd highly welcome a patch that implements this functionality.

[evpobr]

Please no. CMake approach is to make separate builds, you have problems implementing this now because it is wrong way. I did this mistake with libsndfile, no real benefits, but it was very hard to support and use. Don't try to mimic Autotools, it is different in many aspects.