error while building trunk (only for c++) on ubuntu 14.04 LTS

[acc2]

hi.
i am trying to build gcc 6 (since i want to try out some c++17 features)
and get an error:

two systems: ubuntu 14.04 LTS
A: as direct system (svn Revision: 232603)
B: in virtual box   (svn Revision: 232581)

directories:
gcc
 - srcdir (the svn repo)
 - objdir
 - installdir

running:
cd objdir
../srcdir/configure --prefix=/home/qwer/gcc/installdir/ --enable-threads
--enable-link-mutex --enable-languages=c++ --disable-multilib
make profiledbootstrap

the failing command:

on A (using the contrib/download_prerequisites script):
/home/SMBAD/grimm/home/gcc/objdir/./prev-gcc/xg++
-B/home/SMBAD/grimm/home/gcc/objdir/./prev-gcc/
-B/home/qwer/gcc/installdir/x86_64-pc-linux-gnu/bin/ -nostdinc++
-B/home/SMBAD/grimm/home/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/src/.libs
-B/home/SMBAD/grimm/home/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/libsupc++/.libs 
-I/home/SMBAD/grimm/home/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu 
-I/home/SMBAD/grimm/home/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/include 
-I/home/SMBAD/grimm/home/gcc/srcdir/libstdc++-v3/libsupc++
-L/home/SMBAD/grimm/home/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/src/.libs
-L/home/SMBAD/grimm/home/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/libsupc++/.libs
-fno-PIE -c   -g -O2 -fprofile-use -DIN_GCC     -fno-exceptions
-fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing
-Wwrite-strings -Wcast-qual -Wmissing-format-attribute
-Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros
-Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I. -I.
-I../../srcdir/gcc -I../../srcdir/gcc/. -I../../srcdir/gcc/../include
-I../../srcdir/gcc/../libcpp/include
-I/home/SMBAD/grimm/home/gcc/objdir/./gmp
-I/home/SMBAD/grimm/home/gcc/srcdir/gmp
-I/home/SMBAD/grimm/home/gcc/objdir/./mpfr
-I/home/SMBAD/grimm/home/gcc/srcdir/mpfr
-I/home/SMBAD/grimm/home/gcc/srcdir/mpc/src 
-I../../srcdir/gcc/../libdecnumber
-I../../srcdir/gcc/../libdecnumber/bid -I../libdecnumber
-I../../srcdir/gcc/../libbacktrace
-I/home/SMBAD/grimm/home/gcc/objdir/./isl/include
-I/home/SMBAD/grimm/home/gcc/srcdir/isl/include  -o omp-low.o -MT
omp-low.o -MMD -MP -MF ./.deps/omp-low.TPo ../../srcdir/gcc/omp-low.c

on B (using packages from the repo):
/home/qwer/gcc/objdir/./prev-gcc/xg++
-B/home/qwer/gcc/objdir/./prev-gcc/
-B/home/qwer/gcc/installdir/x86_64-pc-linux-gnu/bin/ -nostdinc++
-B/home/qwer/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/src/.libs
-B/home/qwer/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/libsupc++/.libs 
-I/home/qwer/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu 
-I/home/qwer/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/include 
-I/home/qwer/gcc/srcdir/libstdc++-v3/libsupc++
-L/home/qwer/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/src/.libs
-L/home/qwer/gcc/objdir/prev-x86_64-pc-linux-gnu/libstdc++-v3/libsupc++/.libs
-fno-PIE -c   -g -O2 -fprofile-use -DIN_GCC     -fno-exceptions
-fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing
-Wwrite-strings -Wcast-qual -Wmissing-format-attribute
-Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros
-Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I. -I.
-I../../srcdir/gcc -I../../srcdir/gcc/. -I../../srcdir/gcc/../include
-I../../srcdir/gcc/../libcpp/include  -I../../srcdir/gcc/../libdecnumber
-I../../srcdir/gcc/../libdecnumber/bid -I../libdecnumber
-I../../srcdir/gcc/../libbacktrace   -o omp-low.o -MT omp-low.o -MMD -MP
-MF ./.deps/omp-low.TPo ../../srcdir/gcc/omp-low.c

error (same for both modulo home directory):
../../srcdir/gcc/omp-low.c: In function ‘void
expand_omp_target(omp_region*)’:
../../srcdir/gcc/omp-low.c:13345:8: error: ‘i_async’ may be used
uninitialized in this function [-Werror=maybe-uninitialized]
      args.safe_push (oacc_launch_pack
                      ~~~~~~~~~~~~~~~~
        (GOMP_LAUNCH_ASYNC, NULL_TREE, i_async));
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

cc1plus: all warnings being treated as errors
make[3]: *** [omp-low.o] Error 1
make[3]: *** Waiting for unfinished jobs....
rm gcov-tool.pod gpl.pod cpp.pod gfdl.pod fsf-funding.pod gcc.pod gcov.pod
make[3]: Leaving directory `/home/qwer/gcc/objdir/gcc'
make[2]: *** [all-stagefeedback-gcc] Error 2
make[2]: Leaving directory `/home/qwer/gcc/objdir'
make[1]: *** [stagefeedback-bubble] Error 2
make[1]: Leaving directory `/home/qwer/gcc/objdir'
make: *** [profiledbootstrap] Error 2

can someone help and tell me what i am doing wrong?

thanks.
raphael.