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

configure

@@ -568,7 +568,7 @@ fi
 
 : ${make=${MAKE-make}}
 : ${install=${INSTALL-install}}
-: ${python=${PYTHON-python}}
+: ${python=${PYTHON-python -B}}
 : ${smbd=${SMBD-/usr/sbin/smbd}}
 
 # Default objcc to clang if available, otherwise use CC
@@ -1349,7 +1349,7 @@ fi
 
 # Note that if the Python conditional here evaluates True we will exit
 # with status 1 which is a shell 'false' value.
-if ! "$python" -c 'import sys; sys.exit(sys.version_info < (2,4) or sys.version_info >= (3,))'; then
+if ! $python -c 'import sys; sys.exit(sys.version_info < (2,4) or sys.version_info >= (3,))'; then
   error_exit "Cannot use '$python', Python 2.4 or later is required." \
       "Note that Python 3 or later is not yet supported." \
       "Use --python=/path/to/python to specify a supported Python."