Don't require a C++ compiler for libambit and the example
Clarify shell flavour expectations

The default `/bin/sh` on Debian (`dash`) does not have `[`, only
`test`.
Don't require a C compiler for a C++ only GUI application
Document build requirements in an easily verifiable way

Of course, this only cover the next Debian stable release but it is
complete, builds on very minimal assumptions and can be checked for
correctness in a jiffy (less then 30 seconds on my machine building
from scratch).