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.

Change-Id: Ib44a8e358643cf19b4834b3bd4d5b88db6cd0ccf
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>