CP-6530: Add argument --use-distfiles to configure.py

We can now configure whether to use the local distfiles cache.

Signed-off-by: John Else <john.else@citrix.com>
CP-6530: Add CLI arguments to build.py

Whether we are using mock, and whether we are in the XS build system,
are now configurable.

Signed-off-by: John Else <john.else@citrix.com>