configure: Pass `-L/to/nix' as $LDFLAGS, not $LIBS.

Dependencies

• [2] 2KJFOYGS hydra: Actually check for Nix in `configure'.
• [3] WR6LRCAC oops, committed too much
• [4] NDJ6PZB7 * Fix symbol to look for.
• [5] HX4QYOYA add first evaluations tests
• [6] FV2M6MOT hydra: use autoconf/-make

Change contents

• replacement in configure.ac at line 45
  [2.91]→[2.91:113](∅→∅)
  − LIBS="-L$nix/lib/nix"
  [2.91]

  [2.113]
  + LDFLAGS="-L$nix/lib/nix"