raptorjit.nix: preBuild = 'make bootstrapclean'

Start with 'make bootstrapclean' in builds under nix. This removes the
generated code that lives in-tree in the repository to ensure that it
will be rebuilt. This means that the nix build is not dependent on the
in-tree code being up-to-date.

Nix provides all of the build dependencies so there is no issue with
rebuilding the generated code.

raptorjit.nix

@@ -9,6 +9,9 @@ mkDerivation rec {
   name = "raptorjit-${version}";
   inherit version;
   src = source;
+  preBuild = ''
+    make bootstrapclean
+  '';
   buildInputs = [ luajit ];  # LuaJIT to bootstrap DynASM
   installPhase = ''
     mkdir -p $out/bin