Detect ICU from CMake

There is some ICU detection as part of PAL (it checks for `uchar.h`), but ChakraCore main CMake file does not really look for the library, unless `ICU_INCLUDE_PATH` is set.

For 'system ICU` case Linux distros get hardcoded library names, and OSX does not get anything set.

We might need to think what the interaction between EMBED_ICU, SYSTEM_ICU, and ICU_INCLUDE_PATH should be.