Eliminate the need to run CMake again before installing/packaging

[TheAssassin]

Right now, CMake has to be run twice before make install, CPack etc. work. This is due to our private bundling of some libraries, mainly libappimageupdate (and the Qt interface library) and libappimage. CMake doesn't know their path before they're built, and the packaging will omit them unless you run CMake again, which updates some sort of cache, it seems.

A warning has already been implemented reminding users of this need. But we should try to get rid of it entirely. It's just annoying and cumbersome, and also sometimes requires compiling large parts of the projects twice.

For the Lite builds for instance, running twice is completely superfluent, as linuxdeploy will collect the dependencies anyway. So the starting point to improve the situation is to get rid of having to run CMake twice in this build script.

For the regular package builds, the location is known as well, we just have to convince CMake of putting the paths into the install scripts, even if the files don't exist yet during initial configuration. Perhaps some generator expression will do the job.

[alexanderbazhenoff]

error: downloading 'https://dl.bintray.com/boostorg/release/1.69.0/source/boost_1_69_0.tar.gz' failed 404 after 5 retries on make libappimage libappimageupdate libappimageupdate-qt. Any ideas?