make sure NDEBUG is not defined already before defining it

Patch by Diego 'Flameeyes' Petteno flameeyes AA gentoo PP org
Original thread:
Date: Jun 30, 2006 1:09 AM
Subject: [Ffmpeg-devel] [PATCH] Avoid warning on NDEBUG redefinition

Originally committed as revision 5557 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/common.h

@@ -288,7 +288,7 @@ static inline float floorf(float f) {
 
 /* debug stuff */
 
-#    ifndef DEBUG
+#    if !defined(DEBUG) && !defined(NDEBUG)
 #        define NDEBUG
 #    endif
 #    include <assert.h>