Log Message:
libXt: update to 1.3.1.

Alan Coopersmith (11):
      README.md: update & correct links to O'Reilly X Toolkit books
      unifdef ultrix
      unifdef sgi
      unifdef hpux
      unifdef __osf__
      unifdef AIXV3
      unifdef MOTOROLA
      unifdef USG
      unifdef VMS
      unifdef __EMX__
      unifdef AIXSHLIB & SUNSHLIB

Matt Turner (1):
      libXt 1.3.1

Mohamed Akram (1):
      Improve linking with widget libraries on macOS

Thomas E. Dickey (8):
      issue #17: avoid _Static_assert from c++
      issue #18: cross-build issue in util
      issue #19: include <process.h> on Windows for getpid()
      issue #20: c23 boolean conflict
      remove substitution for EXEEXT_FOR_BUILD, not defined in automake
      Add continue_to_dispatch parameter to _XtResourceConfigurationEH()
      eliminate an unnecessary cast
      quiet a couple of unused-parameter warnings