build, don't overwrite autogenerated DEPENDENCIES
build, dist-hook builds libltdl only if needed

Now "make dist" builds libtdl only if libltdlc.la does not exist, never
rebuilding it. But at least we do it only if needed and it works
correctly in parallel.
Merge remote-tracking branch 'rudi/fixes'

* rudi/fixes:
  build, dist-hook builds libltdl only if needed
  build, don't overwrite autogenerated DEPENDENCIES