updated build instructions for developers

[Tom Yu]

I recently expanded (and renamed) a minimal wiki page that was
previously about building krb5 from checked-out source code.  I made
it more generally applicable to other builds (from release tarballs,
etc.).  The page now lists (though not exhaustively) build
prerequisites and provides suggestions about separate build trees and
"maintainer mode".  Hopefully it will be more useful to developers
now.  Please feel free to comment or edit. (You need to register for a
wiki account to make edits.)  The URL is:

    http://k5wiki.kerberos.org/wiki/Building

-- 
Tom Yu
Development Team Leader
MIT Kerberos Consortium