BUILD_SHARED_LIBS is defined in cmake docs (https://cmake.org/cmake/help/v3.0/variable/BUILD_SHARED_LIBS.html) to control this. The reason this is useful is, for example, when FreeBSD (or any other system) has a policy against installing static libs this should be supported by the project itself.

Additionally, there is a confusion resulting in failures like this:
/usr/local/bin/ld: cannot find -ldocopt_s

To avoid such confusion please name both libraries the same way.