AccEventGen.py was using an old, one-off Python module for writing make
dependency files. We have a more appropriate API now. So we use it.
While we were here, we modified file writing to use FileAvoidWrite to
avoid always writing the output files and bumping mtimes.
--HG--
extra : rebase_source : 34ef9badacceb4791f27816c756ad9c95bf4069c
598f54ae2d