configure: disable static linking on Darwin

Static liunking is not supported on Darwin, so hide the corresponding
options when the build machine is Darwin.

Reported-by: Andrea Franceschini <therealmorpheu5@gmail.com>
Reported-by: Bryan Hundven <bryanhundven@gmail.com>
Reported-by: "Titus von Boxberg" <titus@v9g.de>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Compute s390-specific values

CT_DoArchTupleValues() {

    # That's the only thing to override

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

        CT_TARGET_ARCH="s390x"

    fi

}