These are the rules so that older bsd systems and the POSIX standard
system can coexist togather.

  1) If you use select then include "ntp_select.h"
     select is not standard, since it is very system depenedent as to where
     select is defined.  The logic to include the right system dependent
     include file is in "ntp_select.h".
  2) Always use POSIX defintion of strings.  Inlcude "ntp_string.h" instaed
     of <string.h>.
  3) Always include "ntp_malloc.h" if you use malloc.
  4) Always include "ntp_io.h" instead of <sys/file.h> or <fnctl.h> to
     get O_* flags.
  5) Always include "ntp_if.h" instead of <net/if.h>.
  6) Always include "ntp_stdlib.h" instead of <stdlib.h>.
  7) Always define a system identifier for any new system added to the
     machines directory.  The identifier should always start with SYS_!
  8) Define any special defines needed for a system in
     ./include/ntp_machine.h based on system identifier. This file is
     included by the "ntp_types.h" file and should always be placed
     first after the <> defines.
  9) Define any special library prototypes left over from the system
     library and include files in the "l_stdlib.h" file. This file is
     included by the "ntp_stdlib.h" file and should ordinarily be
     placed last in the includes list.
 10) Don't define a include file by the same name as a system include file.
     
  
"l_stdlib.h" can contain any extra definitions that are needed so that 
gcc will shut up.  They should be controlled by a system identifier and 
there should be a seperate section for each system. Really this will
make it easier to maintain. 

See include/ntp_machines.h for the verious compile time options.

Good luck.

Bill Jones, with amendments by Dave Mills