gen_makefile.py: add --auto-distclean

Add a new option that avoids stopping the build with:
  configure: error: source directory already configured; run "make distclean" there first

I run into this frequently, when e.g. running './configure; make release'
inside Osmocom repositories and then attempting to build them with
osmo-dev. It is annoying as it does not fail immediatelly, and I need to
manually go to the source directories where it failed, run 'make
distclean' and then attempt to build again. Then it may fail on another
directory with the same problem.

Check for config.status, as that is also what ./configure checks for
before printing the error.

Change-Id: Iac71333e8c5da8dee4e8f34e2af8db12d2c08021
1 file changed