I am trying to build mainline GCC (revision 160215) configured
with -enable-build-with-cxx .

I get the following error during bootstrap:

/user/inria/fsf/bldxx-20100603/./prev-gcc/g++ -B/user/inria/fsf/bldxx-20100603/./prev-gcc/ -B/user/inria/cxx/i686-pc-linux-gnu/bin/ -nostdinc++ -I/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/include/i686-pc-linux-gnu -I/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/include -I/user/inria/fsf/gcc/libstdc++-v3/libsupc++ -L/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/src/.libs -c  -g -O2 -fomit-frame-pointer -gtoggle -DIN_GCC   -W -Wall -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I. -I. -I../../gcc/gcc -I../../gcc/gcc/. -I../../gcc/gcc/../include -I../../gcc/gcc/../libcpp/include  -I../../gcc/gcc/../libdecnumber -I../../gcc/gcc/../libdecnumber/bid -I../libdecnumber  -DCLOOG_PPL_BACKEND  -I/usr/include/libelf  ../../gcc/gcc/simplify-rtx.c -o simplify-rtx.o
/user/inria/fsf/bldxx-20100603/./prev-gcc/g++ -B/user/inria/fsf/bldxx-20100603/./prev-gcc/ -B/user/inria/cxx/i686-pc-linux-gnu/bin/ -nostdinc++ -I/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/include/i686-pc-linux-gnu -I/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/include -I/user/inria/fsf/gcc/libstdc++-v3/libsupc++ -L/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/src/.libs -c  -g -O2 -fomit-frame-pointer -gtoggle -DIN_GCC   -W -Wall -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I. -I. -I../../gcc/gcc -I../../gcc/gcc/. -I../../gcc/gcc/../include -I../../gcc/gcc/../libcpp/include  -I../../gcc/gcc/../libdecnumber -I../../gcc/gcc/../libdecnumber/bid -I../libdecnumber  -DCLOOG_PPL_BACKEND  -I/usr/include/libelf  ../../gcc/gcc/sparseset.c -o sparseset.o
../../gcc/gcc/simplify-rtx.c: In function ‘rtx_def* simplify_immed_subreg(machine_mode, rtx, machine_mode, unsigned int)’:
../../gcc/gcc/simplify-rtx.c:4878:11: error: variable ‘ibyte’ set but not used [-Werror=unused-but-set-variable]
../../gcc/gcc/simplify-rtx.c:4982:16: error: variable ‘ibyte’ set but not used [-Werror=unused-but-set-variable]
../../gcc/gcc/simplify-rtx.c:5027:11: error: variable ‘ibyte’ set but not used [-Werror=unused-but-set-variable]
/user/inria/fsf/bldxx-20100603/./prev-gcc/g++ -B/user/inria/fsf/bldxx-20100603/./prev-gcc/ -B/user/inria/cxx/i686-pc-linux-gnu/bin/ -nostdinc++ -I/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/include/i686-pc-linux-gnu -I/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/include -I/user/inria/fsf/gcc/libstdc++-v3/libsupc++ -L/user/inria/fsf/bldxx-20100603/prev-i686-pc-linux-gnu/libstdc++-v3/src/.libs -c  -g -O2 -fomit-frame-pointer -gtoggle -DIN_GCC   -W -Wall -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I. -I. -I../../gcc/gcc -I../../gcc/gcc/. -I../../gcc/gcc/../include -I../../gcc/gcc/../libcpp/include  -I../../gcc/gcc/../libdecnumber -I../../gcc/gcc/../libdecnumber/bid -I../libdecnumber  -DCLOOG_PPL_BACKEND  -I/usr/include/libelf  ../../gcc/gcc/sreal.c -o sreal.o
cc1plus: all warnings being treated as errors

make[3]: *** [simplify-rtx.o] Error 1
make[3]: *** Waiting for unfinished jobs....
rm gcj-dbtool.pod jcf-dump.pod jv-convert.pod grmic.pod gcov.pod gcj.pod gc-analyze.pod gfdl.pod cpp.pod gij.pod gcc.pod gfortran.pod fsf-funding.pod
make[3]: Leaving directory `/user/inria/fsf/bldxx-20100603/gcc'
make[2]: *** [all-stage2-gcc] Error 2
make[2]: Leaving directory `/user/inria/fsf/bldxx-20100603'
make[1]: *** [stage2-bubble] Error 2
make[1]: Leaving directory `/user/inria/fsf/bldxx-20100603'
make: *** [bootstrap] Error 2