DCHECK_ALWAYS_ON to make D* enabled under NDEBUG

The macro NDEBUG could be automatically defined for release build on
some build environments (e.g. MSVC).  If we use NDEBUG as a key to
distinguish using DCHECK as CHECK (I call this DCHECK is enabled) or
not, we cannot make DCHECK enabled for release build on such
environments.

Considering people use a program with glog for presubmit testing or
dogfooding, they should need to do release build with DCHECK enabled.