portability of ${foo:+bar}

[Ralf Wildenhues]

Quoting autoconf.info:

| `${VAR:-VALUE}'
|      Old BSD shells, including the Ultrix `sh', don't accept the colon
|      for any shell substitution, and complain and die.

Can I assume that any shell on a system, which has shared libraries,
understands `${foo:+bar}' and `${foo:-bar}' correctly, even if bar
happens to be `:'?

Rationale: it makes avoiding a stray colon in colon-separated variables
sooo much easier, and I don't think Libtool makes use of them for
systems without shared libraries.

Cheers,
Ralf