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.

modules/FindMagnum.cmake

@@ -245,7 +245,7 @@ foreach(component ${Magnum_FIND_COMPONENTS})
         if(${component} STREQUAL GlutApplication)
             find_package(GLUT)
             if(GLUT_FOUND)
-                set(_MAGNUM_${_COMPONENT}_LIBRARIES ${GLUT_LIBRARIES} ${_WINDOWCONTEXT_MAGNUM_LIBRARIES_DEPENDENCY})
+                set(_MAGNUM_${_COMPONENT}_LIBRARIES ${GLUT_glut_LIBRARY} ${_WINDOWCONTEXT_MAGNUM_LIBRARIES_DEPENDENCY})
             else()
                 unset(MAGNUM_${_COMPONENT}_LIBRARY)
             endif()