autoupdate: replace obsolete macros AC_AIX, AC_MINIX, and AC_GNU_SOURCE

- AC_AIX, AC_MINIX, and AC_GNU_SOURCE by AC_USE_SYSTEM_EXTENSIONS
  https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.72/autoconf.html#index-AC_005fAIX
  https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.72/autoconf.html#index-AC_005fMINIX-1
  https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.72/autoconf.html#index-AC_005fGNU_005fSOURCE-1
9 files changed