Python module installation is broken in BIND 9.13.4

The fix for #601 (closed) introduced by !980 (merged) breaks Python module installation due to setting PYTHON_INSTALL_DIR and PYTHON_INSTALL_LIB to unspec and --install-dir=unspec, respectively, when --with-python-install-dir is not used.

This prevents BIND 9.13.4 packages with Python support from being built:

• https://copr-be.cloud.fedoraproject.org/results/isc/bind-dev/epel-7-ppc64le/00827985-isc-bind/builder-live.log
• https://copr-be.cloud.fedoraproject.org/results/isc/bind-dev/epel-7-x86_64/00827985-isc-bind/builder-live.log
• https://copr-be.cloud.fedoraproject.org/results/isc/bind-dev/fedora-27-i386/00827985-isc-bind/builder-live.log
• https://copr-be.cloud.fedoraproject.org/results/isc/bind-dev/fedora-27-ppc64le/00827985-isc-bind/builder-live.log
• https://copr-be.cloud.fedoraproject.org/results/isc/bind-dev/fedora-27-x86_64/00827985-isc-bind/builder-live.log
• https://copr-be.cloud.fedoraproject.org/results/isc/bind-dev/fedora-28-i386/00827985-isc-bind/builder-live.log
• https://copr-be.cloud.fedoraproject.org/results/isc/bind-dev/fedora-28-ppc64le/00827985-isc-bind/builder-live.log
• https://copr-be.cloud.fedoraproject.org/results/isc/bind-dev/fedora-28-x86_64/00827985-isc-bind/builder-live.log

I implemented a workaround in the *.spec file to fix RPM builds but this problem should be properly addressed in the BIND repository.