configure.ac: check support for C11 and C++11 (patch #8906) * configure.ac: two new checks for support of C++11 and C11 versions of the standard and to add required flags to CXX and CC. These need to be done before AC_PROG_CPP and AC_PROG_CXXPP since the later will make use of the defined CXX and CC. * m4/prog_c.m4, m4/prog_cxx.m4: added files for OCTAVE_PROG_CC_C11 and OCTAVE_PROG_CXX_CXX11 which are copied from autoconf source tree implementations of _AC_PROG_CC_C11 and _AC_PROG_CXX_CXX11.

gnulib-hg = gnulib-hg