Replace log2f(10) with a constant

Originally committed as revision 21924 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/acelp_pitch_delay.c

@@ -128,7 +128,7 @@ float ff_amr_set_fixed_gain(float fixed_gain_factor, float fixed_mean_energy,
     // ^g_c = ^gamma_gc * 100.05 (predicted dB + mean dB - dB of fixed vector)
     // Note 10^(0.05 * -10log(average x2)) = 1/sqrt((average x2)).
     float val = fixed_gain_factor *
-        exp2f(log2f(10.0) * 0.05 *
+        exp2f(M_LOG2_10 * 0.05 *
               (ff_dot_productf(pred_table, prediction_error, 4) +
                energy_mean)) /
         sqrtf(fixed_mean_energy);

libavutil/mathematics.h

@@ -35,6 +35,9 @@
 #ifndef M_LN10
 #define M_LN10         2.30258509299404568402  /* log_e 10 */
 #endif
+#ifndef M_LOG2_10
+#define M_LOG2_10      3.32192809488736218171  /* log_2 10 */
+#endif
 #ifndef M_PI
 #define M_PI           3.14159265358979323846  /* pi */
 #endif