Clean the build system and use the new XDT_VERSION_INIT

- XDT_VERSION_INIT is available since xfce4-dev-tools 4.15.0, and
  simplify versions initialization in configure
- We can directly use a configure.ac file now
- update configure.ac syntax
2 jobs for !9 with clean_build_system in 1 minute and 24 seconds
latest detached
Status Job ID Name Coverage
  Build
passed #9626
build

00:00:31

 
  Distcheck
passed #9627
distcheck

00:00:53