5.2: libgee required even without building vala extensions

In 5.2.8 the configure run fails without libgee, even if vala extensions are disabled. The check result fatality is done via a variable disable_vala_ext that's not set anywhere anymore after the vala handling rework, thus configure always fails if libgee wasn't found. Please use the correct variable for the check for any upcoming 5.2 releases, if any.