I was writing a response this AM but our corp. network connection was severed for the day
Anyway, ldconfig is used by older style DYNAMIC loading linkers
to keep a cache (usually /etc/ld.so.cache) of dynamic libraries and symbols.
Darwin doesn't have that and doesn't need it.
What you have are static libraries (the .a's). Even if you had
ldconfig, it wouldn't matter.
Nor am I even certain where your seeing those installation
instructions. AS is built almost totally statically so unless the themes themselves are dynamic executables looking for dynamic runtime libs (which you dont have), there isn't a reason to do what your doing.
The themes instructions I see on afterstep.org are pretty clear.