fix default value of ProcessCommand::m_responseFileThreshold

On Windows it should be around 32000 and everywhere else negative.
A value of 0 means to always use a response file regardless how
long the command line is.

Change-Id: I7c90bb536ca9cb9f5b41fe5dbdb50b27d7057136
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
remove property cpp.responseFileThreshold

This is host-system dependent and does not belong here.
We're relying on a sane default value.
It can still be overridden if needed.

Change-Id: If8137dd1f781a56053d2b9992ba80d561bd1e1ef
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>