Build against latest Nix

Dependencies

• [2] PIMGMGAF Rename hydra_eval_jobs to hydra-eval-jobs
• [3] MIC2O6ZF Use evalFile() instead of parseExprFromFile()
• [4] A4SFSXJW libexpr's symbol table has changed
• [5] NDJ6PZB7 * Fix symbol to look for.
• [6] FJILMA6G configure: Preserve the user's $CPPFLAGS and $LDFLAGS.
• [7] FV2M6MOT hydra: use autoconf/-make
• [8] WR6LRCAC oops, committed too much
• [9] KBONEPPH configure: Pass `-L/to/nix' as $LDFLAGS, not $LIBS.
• [10] 2KJFOYGS hydra: Actually check for Nix in `configure'.
• [11] HX4QYOYA add first evaluations tests
• [12] M3A5PZIH hydra: Clarify the dependency on BDW-GC.

Change contents

• replacement in configure.ac at line 70
  [4.248]→[3.0:72](∅→∅)
  − AC_CHECK_LIB([expr], [_ZN3nix9EvalState8evalFileERKSsRNS_5ValueE], [:],
  [4.248]

  [4.74]
  + AC_CHECK_LIB([nixexpr], [_ZN3nix9EvalState8evalFileERKSsRNS_5ValueE], [:],
• replacement in src/c/Makefile.am at line 4
  [2.77]→[4.216:286](∅→∅),[4.18008]→[4.216:286](∅→∅)
  − hydra_eval_jobs_LDADD = -lmain -lexpr -L$(nix)/lib/nix $(BDW_GC_LIBS)
  [2.77]

  [4.286]
  + hydra_eval_jobs_LDADD = -lnixmain -lnixexpr -L$(nix)/lib/nix $(BDW_GC_LIBS)