configure broke when I tried to merge gnome-extra/gnome-builder-3.18.1 (with +vala set) claiming I didn't have vala installed. Looking through the source code I found that vala >0.29 is required for the vala-pack plugin. Adding VALA_{MIN|MAX}_API_VERSION="0.30" to the ebuild pulled in vala-0.30 as a dep and allowed gnome-builder to merge properly.
[master fb4d0d5] gnome-extra/gnome-builder: vala-0.30 is needed, bug #567224 by Mark R. Pariente 1 file changed, 1 insertion(+)