Try to call Override less

[mwichmann]

Eliminate a kwarg explain from the call to Builder() for Mkdir. Arg is not used (maybe it was in the past, though have not found the evidence for this), which meant we had an override dict in this case. Suprisingly, this ended up in the override logic a lot in a build of a large sample project - over 2000 extra calls to scons_subst_once that weren't needed.

Builder and Executor both have places where they call Override(). While the Override factory indeed returns quickly if the override dict is empty, avoid the function call entirely in this case by checking (there was an old TODO comment to this effect in Builder.__init__.py).

The Executor init method declared multiple paramters with mutable defaults, which means they "persist". While there's no evidence this is calling a real problem, eliminate part of this footgun by defaulting overridelist and builder_kw to None and checking for that (targets and sources still default to a mutable empty list, a TBD perhaps).

There are no user-visible effects to these changes; existing tests have been run and indicate no impacts. Thus, there are no doc or test changes included.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation