Fix mismatch (missing 'const' to mactch funcs declarations).

Was breaking windows compile, reported by bdancer over IRC, thanks.

Also, quite some annoying 'unused vars' warnings (debug-only vars).

source/blender/python/bmesh/bmesh_py_utils.c

@@ -255,6 +255,7 @@ static PyObject *bpy_bm_utils_vert_splice(PyObject *UNUSED(self), PyObject *args
 	/* should always succeed */
 	ok = BM_vert_splice(bm, py_vert->v, py_vert_target->v);
 	BLI_assert(ok == true);
+	UNUSED_VARS_NDEBUG(ok);
 
 	Py_RETURN_NONE;
 }