Update FindICU.cmake to better support Ubuntu and MS Visual Studio
- On Ubuntu, several binaries that FindICU.cmake searches for are installed in 'sbin' instead of 'bin' [1].
- When compiling ICU statically on Windows with Visual Studio, the library file names are prefixed with 's' [2].
[1] https://packages.ubuntu.com/xenial/amd64/icu-devtools/filelist
[2] http://source.icu-project.org/repos/icu/trunk/icu4c/source/icudefs.mk.in
STATIC_PREFIX_WHEN_USED = s