Build: Allow again building MD4C as static lib. * On Windows, we build static lib by default, as there is no /usr/lib counterpart. * On other systems the shared lib is the default. * Use option BUILD_SHARED_LIBS to override it: $ cmake -DBUILD_SHARED_LIBS=ON path_to_root # to build shared lib $ cmake -DBUILD_SHARED_LIBS=OFF path_to_root # to build static lib