terminal feature flag is not wired to the build system

Version information

Thunar 4.21.3 Gentoo Linux/amd64

Current behavior

445abc04 added a vte dependency along with a terminal feature flag. However, the flag isn't actually wired to anything, and Thunar instead uses vte automagically if it happens to be available at build time.

Expected outcome

terminal flag controlling the vte dependency and therefore terminal support.