Bug 256679

When enabling -DENABLE_BUBBLEWRAP_SANDBOX=ON is needed to defined to the build the paths (full-paths) to the bwrap and xdg-dbus-proxy binaries. The current CMake code is auto-detecting those paths by calling CMake function find_program(), so it is defining the paths to those programs with the values from the host system. But when cross-compiling that is wrong because the target binaries end with the values for the paths from the host system which don't necessary have to match the values from the target system. Those value may match if the distro of the host and the target is the same (for example cross-compiling on debian x86_64 for debian aarch64). But in other case don't match (for example cross-compiling in Debian for Yocto) I can't see how it will be possible to auto-detect the value that this programs will have in the target system, so the only sane way of dealing with this seems to be to give an error at configure time and ask for those paths to be defined manually.