cc/gcc: set CXXFLAGS at configure gcc

Since gcc 4.8 C++ is also used as implementation language (see gcc
release notes).

Signed-off-by: "Daniel Dittmann" <ddittmann@gmx.net>
Message-Id: <acc7d11bc77b30f21c5b.1388863298@bernalk.machteam>
Patchwork-Id: 306883

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

    if [ "${CT_ARCH_64}" = "y" ]; then

        CT_TARGET_ARCH="s390x${CT_ARCH_SUFFIX}"

    fi

}