configure.ac: Fail better when interfaces file is missing
configure.ac: Fail better when interfaces file is missing

Check whether the Flatpak interfaces directory is correctly set in the
configure file and the file exists if we're not building from tarball,
rather than failing at the build stage.

This should throw an error if docs have been enabled and Flatpak
development files are absent or too old.

Closes: #450

matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal