Heads-up: in the gretl 1.9.13 release (coming shortly) I'm reversing the
preference order between gtk 2 and gtk 3. Up till now we have "preferred"
gtk 2, but now that gtk 3 is mature we'll switch. That is:
1) If both gtk 3 and gtk 2 are present and no relevant option is given to
./configure, gretl will be built against gtk 3.
2) If you have both and prefer using gtk 2, you should give the option
3) If you only have gtk 2, no problem, it'll get used.
However, note one thing: if you're using gtk 3 you'll also need version
3.0 of gtksourceview (for syntax highlighting).