comerr: compile_et -emit option for parallel make

Add the -emit option to the compile_et command to support parallel make.

The -emit option allows make to generate the header and the source files
independently, instead of building two files at the some time.  This
avoids the issue where one command creates two separate files, which is
difficult to handle correctly for parallel makes.

Reviewed-on: http://gerrit.openafs.org/7921
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 9df43aacab0f311c15837b230761a11750f8b9cb)

Change-Id: Id560fcec356a4b36c0e311440f74a97a670c07d1
Reviewed-on: http://gerrit.openafs.org/11227
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>