Change compile_et to generate header files that use <et/com_err.h>

Change compile_et to generate header files that use <et/com_err.h>
instead of <com_err.h>, so the current version of the header file
is used.

Add a --build-tree option to compile_et to make sure that it uses
the et_?.awk files from the build tree.

Remove legacy support for varargs.h, K&R C, and pre-POSIX signal
support.  Also fixed gcc -Wall nits.