beecrypt: make a config option for C++ support

beecrypt used to pull in icu automatically when C++ support was enabled,
but since icu is pretty large and slow, that automatic dependency was
removed.  However, it is not obvious to the user that he should enable
icu in order to get C++ support in beecrypt. Therefore, make the C++
support a user visible config option and mention icu in the help text.

It would also be possible to automatically enable C++ support if icu
happens to be selected for another reason, but that doesn't really seem
to offer an advantage.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>