- flag for an option to set if changing it will trigger a new 'make depend'
- same for 'make clean'
- Specify files to be removed on clean.
- static compilationa (?)
- Let the dependency index depend on the Makeinfo files
  (What about the internal dependencies among Makeinfo)

some docs
- uqm_LDFLAGS etc are only relevant in the top dir
- some variables are 'inherited' from parent dir
  update: this is not true anymore. The behavior was not needed and
  would just complicate things.
- files are sourced (.)