Force rebuild on cmd change.
Force rebuild on cmd change.

Adds an option '--force-rebuild' which will automatically get set if
the command-line options to build_nexe.py, have changed between this
and the previous invocation.  The previous invocation is stored at the
output name + '.cmd'.

R=bradnelson@google.com, mcgrathr@chromium.org
BUG=https://code.google.com/p/nativeclient/issues/detail?id=3927

Review URL: https://codereview.chromium.org/507123002

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@13668 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client