Log Message:
devel/smake: Update to 1.5nb1

Changelog from AN-2021-07-29:
- smake: changed open(..., 0) -> open(..., O_RDONLY) for z/OS

  Thanks to a report from Matthew R. Wilson

- smake: z/OS is using EBCDIC and for this reason, smake cannot continue
  to #define NWARN 0x4000, as this bit would be in conflict with bits
  needed by the "short int" computed from characters used by macro
  assignment operator characters. e.g. "'=' | '+' << 8"

  smake now uses #define NWARN 0x8000 as this bit is only in use by
  EBCDIC digits that do not appear in the 16 bit values used as macro
  assignment operators inside smake.

  Thanks to a report from Matthew R. Wilson

- smake: A .POSIX: line in the Makefile is now correctly detected
  immediately and switches immediately into POSIX mode. This is
  important in case that the parser needs to behave different already,
  depending on whether it is in POSIX mode or not.

- smake: Since the GNU assignment operator := that has been adopted
  by POSIX as ::= causes unpredictable behavior, in case that += is
  used, it is now only supported when smake is in "POSIX mode", e.g.
  by putting .POSIX: into the Makefile. As this results in modifying
  the parser based on the current POSIX state, this needs the extension
  mentioned above.