ICU configuration is inconsistent across Linux and macOS

[jackhorton]

If ICU can't be found as the user-provided path, the build fails on Linux and falls back to --no-icu on macOS. We should probably choose 1 and stick with it.

[jackhorton]

I believe @obastemur voted to make both fall back to --no-icu

[dilijev]

Seems reasonable.

[obastemur]

+1