configure: Use -B switch only for Python versions which support it

Commit 1d984a67a9 added the -B switch
unconditionally. This breaks Python versions before 2.6 which don't
support that switch.

Now configure adds -B only if it is accepted by the Python interpreter.

This modification introduces a small incompatibility because -B might now
also be added when configure was called with --python=PYTHON_INTERPRETER.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

configure

@@ -579,7 +579,7 @@ fi
 
 : ${make=${MAKE-make}}
 : ${install=${INSTALL-install}}
-: ${python=${PYTHON-python -B}}
+: ${python=${PYTHON-python}}
 : ${smbd=${SMBD-/usr/sbin/smbd}}
 
 # Default objcc to clang if available, otherwise use CC
@@ -1400,6 +1400,13 @@ if ! $python -c 'import sys; sys.exit(sys.version_info < (2,4) or sys.version_in
       "Use --python=/path/to/python to specify a supported Python."
 fi
 
+# The -B switch was added in Python 2.6.
+# If it is supplied, compiled files are not written.
+# Use it for Python versions which support it.
+if $python -B -c 'import sys; sys.exit(0)' 2>/dev/null; then
+  python="$python -B"
+fi
+
 if test -z "${target_list+xxx}" ; then
     target_list="$default_target_list"
 else