Add missing floorDivInt test
Add a universal reference constructor to BoxedSet
my old one was missing a std::move and did not accept lvalues.
Merge pull request #235 from undingen/floorDivIntTest

Add missing FloorDivInt test
Add makefile option to defer to cmake
Allow specifying a custom path to ninja
Raise this as an ImportError instead of SystemError

With the recent package import support, we broke importing optparse.
This is because we now can import "encodings" (a package), which imports
codecs, which raises a SystemError when trying to import _codecs which
we don't support yet.

This was getting ignored before since optparse wrapped this process
(which happens via "import gettext") in a try-except and catches the
ImportError.