Fixed the WNCK_CHECK_VERSION calls to only be used when GTK is version 3.0

mate-panel fails to build unless GTK is version 3 due to the WNCK_CHECK_VERSION macro. This macro requires libwnck3 which is only used in the build if GTK 3 is also used. This patched version checks if GTK is version 3 using the GTK_CHECK_VERSION macro and then proceeds to check the WNCK_CHECK_VERSION macro.