Introduce a new lp.cdefs.h file, which will be used to make it
somewhat easier to build this lpr on other operating systems.
This simply includes <sys/cdefs.h> when that is appropriate,
and then checks for any cdefs-ish macros that lpr uses, and
defines them if they don't already exist. This is only a start
at making freebsd's lpr less of a hassle to port. It is mainly
added so all of lpr can be changed to use the __FBSDID() macro,
without making it *more* of a hassle to build on other OS's.

Reviewed by: discussed with bde and obrien
MFC after: 15 days