[qt5-build.eclass] Use usex operator.

Maybe you want to give usex operator a try here? 

"USAGE: usex 〈USE flag〉 [true output] [false output] [true suffix] [false suffix]
DESCRIPTION:
If USE flag is set, echo [true output][true suffix] (defaults to "yes"),
 otherwise echo [false output][false suffix] (defaults to "no")."