hydra: Actually check for Nix in `configure'.

Dependencies

• [2] FV2M6MOT hydra: use autoconf/-make

Change contents

• edit in configure.ac at line 4
  [2.16027]

  [2.16027]
  +

  + AC_LANG([C++])
• edit in configure.ac at line 30
  [2.16382]

  [2.16382]
  +

  + old_CPPFLAGS="$CPPFLAGS"

  + old_LIBS="$LIBS"

  +

  + CPPFLAGS="-I$nix/include/nix"

  + LIBS="-L$nix/lib/nix"

  +

  + AC_CHECK_HEADER([store-api.hh], [:],

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

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

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

  +

  + CPPFLAGS="$old_CPPFLAGS"

  + LIBS="$old_LIBS"