utfcpp 2.3.4

UTF-8 with C++ in a portable way

This is a source only formula, which does not install any binaries, need by the ledger formula.
Since I was uncertain whether utfcpp should be a standalone formula or integrated into the ledger
formula I consulted #machomebrew on IRC and @mistydemeo answered either way would be fine.

I decided to create a separate formula, since utfcpp is a project in its own - even though I don't know
of any other software that uses it apart from ledger - and people developing C++ may find it useful.