modules: actually detect the MAGNUM_PLUGINS_DIR.

It now defaults to magnum/ subdirectory of dir where main Magnum library
was found. It was *annoying* to change /usr/local/lib/magnum to
/usr/lib/magnum _every time_.
modules: work around GLUT_Xi_LIBRARY-related CMake errors on Ubuntu.

These libraries are not needed for GlutApplication, thus we link only to
GLUT_glut_LIBRARY now. Hopefully this won't break linking somewhere
else.