+ improve manpage description for addch versus unctrl format used for
  non-printable characters.
+ modify version-check for gcc/g++, now works for msys2.
+ modify check in _nc_write_entry() for multiply defined aliases to
  report problems within the current runtime of tic rather than for
  conflicts with pre-existing terminal descriptions.
+ allow for MinGW32-/64-bit configurations to use _DEFAULT_SOURCE
+ clarify interaction of -R option versus -C, -I and -r in infocmp
  manpage.
+ build-fix in lib_win32con.c (cf: 20230211).
+ add/use configure check for clock_gettime(), to supersede