libayatana-appindicator-glib fails to cross build from source, because
its upstream build system hard codes the build architecture vapigen. It
correctly discovers the host's vapigen.pc package configuration, but
does not consume the provided vapigen location to discover the correct
vapigen executable path. I'm attaching a patch that correct this. Please
consider applying it and forwarding it upstream as it makes
libayatana-appindicator-glib cross buildable.

Helmut