Regenerate OSX 10.6 with generation 2 scansys.

The pickier check for "header does not preprocess" turned up another
bit of bad data: OSX deprecates <ucontext.h> and issues an #error
if you don't #define _XOPEN_SOURCE before including it.  That is
now [special] entry number two in prereqs.ini! Let's all give Apple
a big hand.

Unfortunately this means we cannot safely impute 10.8 from 10.6;
the header may be completely gone there.