Flatpak: Use ScmRevGen to generate metainfo XML

CMake: Optional system library fixes

Some generators (like Unix Makefiles and Xcode) copy an app's Info.plist at configure time.
This causes a problem when we need to generate the Info.plist at build time, like how we
currently do it with ScmRevGen. Instead of generating the Info.plist directly in ScmRevGen,
provide an Info.plist without any version information to CMake at configure time, have
ScmRevGen generate a separate plist file with the version information at build time, and
then merge the two together to create the final Info.plist.

Because we were setting it with a scope, it wasn't making its way into called functions that would try to inspect it.  Now it does.

For whatever reason, the previous way would inject backslashes into any path that has spaces.