I just check on my linux (Centos):

pkg-config --modversion icu
4.2.1
pkg-config --modversion icu-io
Package icu-io was not found in the pkg-config search path.
Perhaps you should add the directory containing `icu-io.pc'
to the PKG_CONFIG_PATH environment variable
No package 'icu-io' found

Not sure it is going to work. Also --exists returns true for icu, false for icu-io.