oops, committed too much

Dependencies

• [2] HX4QYOYA add first evaluations tests
• [3] FV2M6MOT hydra: use autoconf/-make
• [4] 2KJFOYGS hydra: Actually check for Nix in `configure'.

Change contents

• replacement in configure.ac at line 39
  [3.248]→[2.0:173](∅→∅)
  − #AC_CHECK_LIB([expr], [_ZN3nix17parseExprFromFileERNS_9EvalStateESs], [:],

  − # [AC_MSG_ERROR([Nix library not found; please install Nix or check the `--with-nix' option.])])
  [3.248]

  [3.419]
  + AC_CHECK_LIB([expr], [_ZN3nix17parseExprFromFileERNS_9EvalStateESs], [:],

  + [AC_MSG_ERROR([Nix library not found; please install Nix or check the `--with-nix' option.])])