configure.ac: Declare we're using the serial test harness

Dependencies

• [2] QNX2BXVA hydra: Improve `AC_INIT' call.
• [3] 2KJFOYGS hydra: Actually check for Nix in `configure'.
• [4] UHMUHQMU hydra: fix tarball build, add pre suffix to tarballs
• [5] FV2M6MOT hydra: use autoconf/-make

Change contents

• replacement in configure.ac at line 3
  [4.73]→[4.15999:16027](∅→∅),[2.138]→[4.15999:16027](∅→∅),[4.15999]→[4.15999:16027](∅→∅)
  − AM_INIT_AUTOMAKE([foreign])
  [2.138]

  [3.0]
  + AM_INIT_AUTOMAKE([foreign serial-tests])