Documentation/website suggestion: link to OS X Intel build instructions

[Joshua Dunfield]

Currently, the gmplib.org top-level page says:

  [2006-05-04] GMP does not build on MacInteltosh machines. No fix planned for GMP 4.x.

It's true that the unpatched gmp-4.2.1 does not build on Intel-based
Mac OS X systems; however, a patch is available.  Moreover, straightforward
instructions for applying that patch and successfully building GMP (and
passing "make check", naturally) can be readily downloaded at:

  http://images.apple.com/acg/pdf/MP_Floating_Point_20070313.pdf

Since the only real problem with GMP on Intel-based OS X is finding
that PDF, the statement "No fix planned for GMP 4.x" is misleading.
It would be more accurate, and much more helpful, to say something like

  [2007-xx-xx] Building GMP on Intel-based Mac OS X systems requires a patch.
               Complete instructions can be found in <A HREF="http://images.apple.com/acg/pdf/MP_Floating_Point_20070313.pdf">this PDF</A>.