tree-ssa branch: Fortran libraries don't build

[Joe Buck]

When trying to bootstrap the tree-ssa branch on a Red Hat 7.2 i686 system,
the build of libI77 fails in the configure step.  The top-level configure
line is

/u/jbuck/gnu/src/cvs/tree-ssa/gcc/configure \
    --prefix /usr/localdisk/gcc-cvs/tree-ssa \
    --with-as=/usr/localdisk/binutils-2.13.1/bin/as \
    --with-ld=/usr/localdisk/binutils-2.13.1/bin/ld \
    --enable-languages=c,c++,objc,f77

However, when the build (from make bootstrap-lean) attempts to configure
in libI77, we get

configuring in libI77
running /bin/sh /u/jbuck/gnu/src/cvs/tree-ssa/gcc/libf2c/libI77/configure  --host=i686-pc-linux-gnu --build=i686-pc-linux-gnu --enable-multilib --prefix --with-as=/usr/localdisk/binutils-2.13.1/bin/as --with-ld=/usr/localdisk/binutils-2.13.1/bin/ld --enable-languages=c,c++,objc,f77 --with-gcc-version-trigger=/u/jbuck/gnu/src/cvs/tree-ssa/gcc/gcc/version.c --with-libbanshee --with-target-subdir=i686-pc-linux-gnu --cache-file=../../config.cache --srcdir=/u/jbuck/gnu/src/cvs/tree-ssa/gcc/libf2c/libI77
configure: error: expected an absolute directory name for --prefix: --with-as=/usr/localdisk/binutils-2.13.1/bin/as

Notice that the value of the prefix option has been dropped, so the
--with-as switch is assumed to be the value of the prefix option.
This problem does not happen on the trunk or on the 3.3 branch (for
the identical configure line).