compile_et macros to generate source and headers separately

Common makefile macros to generate headers and source files
separately using the new compile_et -emit option.

Change-Id: I5848922fbe1d94f9d3cec829a3ed657e7f63d191
Reviewed-on: http://gerrit.openafs.org/10869
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>