[tlbuild] tl18 trials

[Karl Berry]

According to grep, strnlen is used in only one place in zziplib, namely
the _zzip_strndup function defined in
zziplib/zziplib-src/zzip/__string.h, only in the case where strndup is
not available (lines 17ff).

Thus I surmise that, for our purposes, it will do well enough to define
the strnlen replacement in that same #else block that defines
_zzip_strndup.

Also should be reported upstream, since evidently they should be
checking for strnlen as they do for strndup.  --thanks, karl.