Skip generation of version and impala-ir .cc files in buildall.sh if -noclean is specified.

Before this patch the -noclean option had almost no effect on the BE build time because
some source files were re-generated with .py scripts regardless.
This change allows ./buildall -skiptests -noclean to do a true incremental rebuild.

Change-Id: Ib3af85db05bdc96a2279a22c1d49d735f2cabd4e
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1394
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins