Our LLVM packages typically built without dump support (which is meant for
developers, not for actual LLVM-based products), and ``void dump() const''
sprinkled across LLVM header files are just prototypes. However, this is
not the case with ``$localbase/llvm$ver/include/llvm/Support/CFGUpdate.h'',
where it contains some actual code:

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
  LLVM_DUMP_METHOD void dump() const { print(dbgs()); }
#endif

This, in turn, results in the linker failure due to "undefined reference to
llvm::cfg::Update<llvm::BasicBlock*>::dump() const". Avoid this by defining
-DNDEBUG when compiling two files which end up having a reference to dump().

This is a prerequisite to enable building with LLVM backend.