Previously there was a variable CGAL_SHARED_LIBS defined by
CGALConfig.cmake. This one is now unified with CGAL_BUILD_SHARED_LIBS.
And the option CGAL_BUILD_SHARED_LIBS is only defined as an option if one
are building CGAL libraries. If CGAL_Common.cmake is included from another
project, such as from UseCGAL.cmake, then this variable is set in
CGALConfig.cmake as a regular (non-cached) CMake variable.