Build: replace Blender specific DEBUG by standard NDEBUG

NDEBUG is part of the C standard and disables asserts. Only this will
now be used to decide if asserts are enabled.

DEBUG was a Blender specific define, that has now been removed.

_DEBUG is a Visual Studio define for builds in Debug configuration.
Blender defines this for all platforms. This is still used in a few
places in the draw code, and in external libraries Bullet and Mantaflow.

Pull Request: https://projects.blender.org/blender/blender/pulls/115774

source/blender/python/mathutils/mathutils_Matrix.h

@@ -20,7 +20,7 @@ typedef unsigned short ushort;
 
 /* matrix[row][col] == MATRIX_ITEM_INDEX(matrix, row, col) */
 
-#ifdef DEBUG
+#ifndef NDEBUG
 #  define MATRIX_ITEM_ASSERT(_mat, _row, _col) \
     (BLI_assert(_row < (_mat)->row_num && _col < (_mat)->col_num))
 #else