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

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

Commit 1d984a67a95d88f3e708b077dab8adeb47c38c93 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>