define some math constants so as not to depend on _XOPEN_SOURCE

Originally committed as revision 14725 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/mathematics.h

@@ -22,8 +22,22 @@
 #define FFMPEG_MATHEMATICS_H
 
 #include <stdint.h>
+#include <math.h>
 #include "rational.h"
 
+#ifndef M_E
+#define M_E            2.7182818284590452354   /* e */
+#endif
+#ifndef M_LN2
+#define M_LN2          0.69314718055994530942  /* log_e 2 */
+#endif
+#ifndef M_LN10
+#define M_LN10         2.30258509299404568402  /* log_e 10 */
+#endif
+#ifndef M_SQRT1_2
+#define M_SQRT1_2      0.70710678118654752440  /* 1/sqrt(2) */
+#endif
+
 enum AVRounding {
     AV_ROUND_ZERO     = 0, ///< round toward zero
     AV_ROUND_INF      = 1, ///< round away from zero