* Ensure that BUILT_SOURCES depends on OBJ_DIR/Makefile so that they do not
  get built before the Makefile is updated.
* Fix build script for yacc & lex files by stopping it from thwarting the
  dependencies on the file. If the .y file changes, it needs to be rebuilt.
  This also cleans up the problem with llvmAsmParser ALWAYS rebuilding its
  Yacc files just to throw them away because there's no change.

llvm-svn: 17288