Fix finding _IceTransNoListen.

This is a patch carried by FreeBSD for a long time; I may have submitted it upstream before.

libICE is **not** in the standary library search paths on FreeBSD, so using the straight-up name *ICE* in `check_library_exists()` won't find it -- even though libICE has previously been found, probably by `find_package(X11)`

Use the full path, which is what `${X11_ICE_LIB}` should hold.