mathematics.h: remove a couple of math defines

While these defines are not defined by the C standard they are
standardized as X/Open System Interfaces Extension. We use the
appropiate _XOPEN_SOURCE define to make them available. They
seem to be available on all FATE configs since the constants
are used in files where mathematics.h is not included.

libavutil/mathematics.h

@@ -26,30 +26,12 @@
 #include "attributes.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_LOG2_10
 #define M_LOG2_10      3.32192809488736234787  /* log_2 10 */
 #endif
 #ifndef M_PHI
 #define M_PHI          1.61803398874989484820   /* phi / golden ratio */
 #endif
-#ifndef M_PI
-#define M_PI           3.14159265358979323846  /* pi */
-#endif
-#ifndef M_SQRT1_2
-#define M_SQRT1_2      0.70710678118654752440  /* 1/sqrt(2) */
-#endif
-#ifndef M_SQRT2
-#define M_SQRT2        1.41421356237309504880  /* sqrt(2) */
-#endif
 #ifndef NAN
 #define NAN            (0.0/0.0)
 #endif