fix to compile builtin-attrs.def on darwin

[Neil Booth]

Aldy Hernandez wrote:-

> i would really like to be able to bootstrap C on a vanilla darwin box
> without having to jump any special hoops, just: 
> 
> 	(configure --enable-languages=c; make bootstrap)
> 
> here is a patch so builtins.c compiles.  it's pretty obvious.  is this ok?
> 
> tangent: perhaps we should force "cc -no-cpp-precomp" when on darwin.  right
> now only bootstrapping in the gcc directory will send -no-cpp-precomp (through
> t-file magic).

Isn't that better than uglifying random parts of GCC all over the
place?  Or is it more than a precompiled header issue?

Neil.