[krbdev.mit.edu #5657] (Mac-specific) PROG_LIBPATH build fix

[Ken Raeburn via RT]

> We need this change when we build for Mac OS X, but this probably
> isn't appropriate for Kerberos in general.  Can this be
> conditionalized?

What's the intent here?  Remove -L options always on the Mac, or specifically avoid -L/usr/lib?  
The latter would make sense as a general change, but would involve a little more code.