While working on growing the fastermake backend to do compilation, I ran into
the problem that there's an implicit dependency from AccEventGen.py on the fact
that IDL files are installed in $DIST/idl. While we may need some way to
express such things in general, it turns out that for this specific use case,
we don't actually /need/ IDLs in $DIST/idl, and a couple paths are enough to
get things to work.
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.
There is only 1 caller to AccEventGen.py in the tree. We don't need
optional arguments. Convert them to positional.
Since the arguments are no longer optional, branches related to their
presence have been removed.
