configure: Preserve the user's $CPPFLAGS and $LDFLAGS.

Dependencies

• [2] KBONEPPH configure: Pass `-L/to/nix' as $LDFLAGS, not $LIBS.
• [3] 2KJFOYGS hydra: Actually check for Nix in `configure'.
• [4] FV2M6MOT hydra: use autoconf/-make

Change contents

• replacement in configure.ac at line 44
  [3.61]→[3.61:91](∅→∅),[3.91]→[2.0:25](∅→∅)
  − CPPFLAGS="-I$nix/include/nix"

  − LDFLAGS="-L$nix/lib/nix"
  [3.61]

  [3.113]
  + CPPFLAGS="$CPPFLAGS -I$nix/include/nix"

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