configure: Don't write .pyc files by default (python -B)

configure: Don't write .pyc files by default (python -B)

When a Python script is run, Python normally writes bytecode into a .pyc file.
QEMU's build process uses several Python scripts which are called from
configure or make.

The generated .pyc files take disk space without being of much use, because
those scripts are short, not time critical and only called a few times.

Python's option -B disables writing of .pyc files. QEMU now uses "python -B"
as default, but it is still possible to choose a different call by passing
--python=PYTHON to configure.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>