[tlbuild] Building on El Capitan

[Akira Kakuto]

> Snow Leopard is known to be affected by the same
> problem as Solaris 10, so a proper fix needs to be
> committed first for that one to work.

In r46771, I have changed to use my_strnlen in all platforms
without strndup. Note that we use strndup only in zziplib.test.
It is not used in our libraly itself.
Karl will change more desirably if he finds it necessary.

Best,
Akira