Related: xfce4-dev-tools#41

- Their gir files have been renamed with a capital has a first letter,
  so adjust garcon to find correct files.
- Update configure.ac.in syntax

Use new xfce_spawn API from libxfce4ui 4.15.6.
This means that items launched via garcon will be reparented to init as
opposed to being launched as child processes.

As agreed on the 4.16 roadmap

The commit and discussion around it gave no reason why 1.16 is required.

All other repos have 1.8 to 1.11 as requirement.
In theory the version could be relaxed further.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

This also means we need to bump our dependency to libxfce4ui to version
4.15.2, which contains the new widget.