Fix compilation error with debug SCons

For some reason SCons defines _DEBUG, not DEBUG as mathutils was
expecting it to be.

Made it so mathutils checks for NDEBUG which mimics BLI_assert
define.

source/blender/blenlib/BLI_math_base.h

@@ -255,7 +255,7 @@ double double_round(double x, int ndigits);
 /* asserts, some math functions expect normalized inputs
  * check the vector is unit length, or zero length (which can't be helped in some cases).
  */
-#ifdef DEBUG
+#ifndef NDEBUG
 /* note: 0.0001 is too small becaues normals may be converted from short's: see [#34322] */
 #  define BLI_ASSERT_UNIT_EPSILON 0.0002f
 #  define BLI_ASSERT_UNIT_V3(v)  {                                            \