In D4501#102244 brecht wrote

If it's not too complex, we would prefer the CMakeLists.txt that
compiles draco itself to be a custom one that use Blender
functions like blender_add_lib. An example of this is in
extern/glog/CMakeLists.txt.

The reason is that we want all libraries to compiled with the
same compiler flags, debug options and so on. If there is a
draco project embedded in the blender project with its own
mechanism it's harder for us to ensure it works as we need
it on all platforms.