swis.h only re-exported if it has changed since the last export.
Stewart Brodie authored
Detail:
  The Makefile now performs a diff of CLIB:h.swis and derived.swis
    in order to determine whether it needs to export the header or
    not.  This can save compilation time if the header file has not
    changed since it used to cause virtually all C code to recompile.
  s/makeswish now imports Hdr:Machine.<Machine> and executes $GetIO
    to ensure that the Portable module header export functions
    correctly.
Admin:
  Tested.

Version 4.98. Tagged as 'RISC_OSLib-4_98'
ac787e1d