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.