avcodec/mpegaudio: use predefined M_SQRT2

M_SQRT2 is defined in math.h, or in avutil/mathematics.h for
compatibility hacks. This uses this value instead of an ad-hoc define.

Fixed point values produced by FIX() remain identical.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavcodec/mpegaudioenc_template.c

@@ -244,11 +244,11 @@ static void idct32(int *out, int *tab)
     do {
         int x1, x2, x3, x4;
 
-        x3 = MUL(t[16], FIX(SQRT2*0.5));
+        x3 = MUL(t[16], FIX(M_SQRT2*0.5));
         x4 = t[0] - x3;
         x3 = t[0] + x3;
 
-        x2 = MUL(-(t[24] + t[8]), FIX(SQRT2*0.5));
+        x2 = MUL(-(t[24] + t[8]), FIX(M_SQRT2*0.5));
         x1 = MUL((t[8] - x2), xp[0]);
         x2 = MUL((t[8] + x2), xp[1]);
 

libavcodec/mpegaudiotab.h

@@ -33,8 +33,6 @@
 #include <stdint.h>
 #include "mpegaudio.h"
 
-#define SQRT2 1.41421356237309514547
-
 static const int costab32[30] = {
     FIX(0.54119610014619701222),
     FIX(1.3065629648763763537),