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

[Ken Raeburn via RT]

[austinj - Fri Aug 24 23:06:11 2007]:
> The intent is specifically to avoid -L/usr/lib, which is already in
> the default search path and
> sometimes needs to be overridden.

Okay.  The patch I just checked in on the trunk seems to do the trick for me in a couple simple 
tests.  Please let me know if it looks good to you.