It's a hard balance between "most people want to run configure --bootstrap" and
"here's a massive wall of text of every detail about building". I think the
balance in this change is better than it was before -- most people on Windows
don't want to build their own binary.
Fixes#983.
This allows trouble-free builds on platforms with non-standard Python
executable names. As a bonus, it passes the arguments received on to
configure.py, so you don't have to reconfigure to build in a
non-standard manner.
