Failing to build gcc 4.7.1

["Philip Boulton" <philipb@ssimicro.com>]

Working on FreeBSD8.1, in a jailed environment.  I have an ancient
(gcc3.4.6) version of gcc that works as my C compiler.  gmake version 3.8.1

I do the following, as root:
> tar zxf gcc-4.7.1.tar.gz 
> cd gcc-4.7.1
> ls -atlG contrib/downl*
> ./contrib/download_prerequisites    <-- seems to work
> ./contrib/download_ecj              <-- seems to work
>  cd ..
>  mkdir obj_gcc471
>  cd obj_gcc471
>  ../gcc-4.7.1/configure     <-- runs successfully.
>  gmake

During the make process, I get the following error:

checking for math/rint... yes
checking for long double... yes
checking format of `long double' floating point... IEEE extended, little
endian
checking for gmp.h... no
configure: error: gmp.h can't be found, or is unusable.
gmake[2]: *** [configure-stage1-mpfr] Error 1
gmake[2]: Leaving directory `/usr/src/local/obj_gcc471'
gmake[1]: *** [stage1-bubble] Error 2
gmake[1]: Leaving directory `/usr/src/local/obj_gcc471'

N.B., gmp.h is *not* present anywhere in gcc-4.7.1 after the
download_prerequisites.  Should it be?

Any help is appreciated,

Thanks.

PB