kauth-gen-policy appears to be a tool for turning a text file in
qsettings format (similar to .ini) into an xml file. Both are textual,
so it seems like putting kauth-gen-policy into a Multi-Arch: foreign
package (like kconfig_compiler_kf5 in #887452).

In a cross compilation environment, kauth-gen-policy is called from the
host system and not from the target system, so a arch depdend path
(/usr/lib/<triplet>) do not make sense in that case and a arch indepdend
should be used. As we do not want to break other usecases we used the
non indrudive way possible, to change this to an option, so we can
modify the path with a camke option.

The bigger picture is to fix libexec path in extra-cmake-modules to a
arch-independed path, as recommended in the FHS 3.0.

887452 https://bugs.debian.org/887452

Bug-Debian: https://bugs.debian.org/887521