icu: tweak icu-config exec_prefix output

Closes #3259

We need to tweak icu-config's exec_prefix too, otherwise if the host
system lacks icu the build fails when looking for the libraries in
/usr/lib rather than the staging directory.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>