[Agda] OS X Installation Instructions Updated

[Shin-Cheng Mu]

Hi,

Due to some changes in my job, I was able to lay my
hands on a MacBook on which I can install everything from
scratch. So I tried to install Agda and updated
the wiki page accordingly:

   http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Main.MacOSX

Thanks to the efforts of Nisse and other people, the
installation is much easier than when I wrote the previous
version last year.

Please revise the page if you spot some mistakes,
and let me know if it does not work for you. :)

sincerely,
Shin-Cheng Mu