The USE variable is passed to configure, the CHOST variable is passed to gcc/g++ If you need it compiled with a certain option, check if the option is available as a USE flag:
emerge lirc -pv
Should list all the options you can pass to it with USE. If that option is not listed, you can either create your own ebuild and emerge it, OR you can configure/make install the package, and then inject the version you installed to fake the system into thinking IT installed it, not you
Cool