scons: bump c++ standard to 14 to match meson

Closes: mesa/mesa#1845
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <mesa/mesa!3500>
31 jobs for !3500 with scons-bump-c++ in 7 minutes and 49 seconds (queued for 4 seconds)
merge request