build: Make error on older GTK+ nicer
build: Make error on older GTK+ nicer

"configure: error: GTK+-3.0 is required to compile gnome-themes-standard"

But I have something even newer than that! And it's definitely
installed, I hear you cry. Yeah, it wasn't you, it was us. No, really,
it was us.

"configure: error: GTK+ 3.1.18 is required to compile gnome-themes-standard"

rgcjonas/adwaita-semidark,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,magcius/gnome-themes-standard,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard,magcius/gnome-themes-standard,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,cosimoc/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,cosimoc/gnome-themes-standard,magcius/gnome-themes-standard,ChrisCummins/gnome-themes-standard