incremental building in 4.2

[Paolo Bonzini]

Mike Stump wrote:
> Someone seriously broke incremental building along to the way to 
> gcc-4.2.  This patch fixes it:

Ok, but please also change this

 >      test [$]$1=$2 && $1=

to this:

        test [$]$1 = $2 && $1=


 >      test [$]$1=$2 && $1=
 > -    ac_cv_path_$1=[$]$1
 > +    test -n "[$]$1" && ac_cv_path_$1=[$]$1

Thanks.

Paolo