gcc 2.96: A configure fix to replace "$(...)" with "${...}"

[Martin v. Loewis]

>  The following patch allows constructs such as `./configure
> --libdir='${exec_prefix}'/<path>' work as expected.  The patch was
> generated from the binutils tree but it applies cleanly against gcc 2.96
> as of May 1st. 

Thanks for your patch. Please send patches to gcc-patches@gcc.gnu.org;
maintainers often cannot catch up with gcc-bugs to review all the
patches. Also, don't forget to include a ChangeLog entry.

Regards,
Martin