A lot of checks and a major workaround are for ICU 4.8 or earlier which
can be removed.
An annoying bug (Parsing of timezones like "GMT" in "full" format followed by unrelated text)
fixed in 4.8.1 is worth avoiding by requiring this version over 4.8.0.
StringPiece is only available in 4.2+ and is required for proper parsing
and formatting.
As this version is now "old enough", just assume 4.2+ removing many
conditionals.