[Unison-hackers] build fails on Mac OS X

[Benjamin Pierce]

> ./unison -doc install > INSTALLATION
> /bin/sh: line 1: ./unison: No such file or directory
> make: *** [INSTALLATION] Error 127
> 
> There was no other error before, so I guess that it's the "./unison"  name
> that is wrong. Should it be
> "./uimac/build/Unison.app/Contents/MacOS/Unison"?

Probably easier just to put a test around it so that it only executes when
the ./unison binary actually exists.  (If someone reminds me the gmake
incantation for this, I can put it in.)

    - B