Unbreak after r436971.

After the kdelibs update to 4.11.30 the CMake policies used by kdelibs-based
projects got stricter.

• Fix `make configure'. The upstream git repository has a add_subdirectory(doc) call, but the code generating the release tarball also added a macro_optional_add_subdirectory(doc) call that was causing CMake to error out due to the same directory being added twice to the build system.
• Fix `make build' with the MUSEPACK option. The FindMuse.cmake patch from r346999 was hardcoding MUSE_LIBRARIES the wrong way. The find_library() call should have been kept, otherwise the linker is passed "-lmpcdec" instead of "/full/path/to/libmpcdec.so".