don't configure libobjc on Darwin

[Nicola Pero]

> >> The Apple compiler has for a long time had a local patch to libobjc to
> >> build but not install it.  This patch prevents both building and
> >> installation on Darwin.  (I believe that if you did actually try to
> >> use the GNU libobjc with the Darwin compiler, you'd discover that
> >> it didn't work anyway---but haven't checked).
> >
> > I'm confused.  Can you explain why this patch is a good idea, in your
> > opinion ?
> 
> If you try to actually build and install FSF GCC on Darwin, you get a 
> non-working objc compiler, because it will default to the NEXT runtime 
> but will be installed with the GNU one in $prefix/lib masking the NEXT 
> one in /usr/lib.

Hmmm.  Ok.

Maybe instead of not compiling/installing it at all, you could compile it,
and install it in some location which doesn't conflict with the NeXT
runtime, and which is not in the standard search paths (so that it's
ignored unless appropriate command line flags are used, and the NeXT
runtime - which is in the standard search paths - is used by default).

While I know it's not that important, and the fact that it works out of
the box for the average user is more important, it still looks a bit of a
waste not to even compile the GNU ObjC runtime on Darwin.  I'm pretty sure
a few power users might come up with some reasons they want/need it.

Compiling it, and installing it in some non-standard (but documented)
location where power users can find and use it, but which doesn't confuse
gcc builds of normal users, might be a solution.