lavu/libm: misc, minor changes

Addition of comments marking the end of ifdef blocks, correction of an
incorrect (at double precision) M_LN2, removal of an unnecessary undef.
Signed-off-by: NGanesh Ajjanagadde <gajjanagadde@gmail.com>

libavutil/libm.h

@@ -38,31 +38,31 @@
 #if !HAVE_ATANF
 #undef atanf
 #define atanf(x) ((float)atan(x))
-#endif
+#endif /* HAVE_ATANF */
 
 #if !HAVE_ATAN2F
 #undef atan2f
 #define atan2f(y, x) ((float)atan2(y, x))
-#endif
+#endif /* HAVE_ATAN2F */
 
 #if !HAVE_POWF
 #undef powf
 #define powf(x, y) ((float)pow(x, y))
-#endif
+#endif /* HAVE_POWF */
 
 #if !HAVE_CBRT
 static av_always_inline double cbrt(double x)
 {
     return x < 0 ? -pow(-x, 1.0 / 3.0) : pow(x, 1.0 / 3.0);
 }
-#endif
+#endif /* HAVE_CBRT */
 
 #if !HAVE_CBRTF
 static av_always_inline float cbrtf(float x)
 {
     return x < 0 ? -powf(-x, 1.0 / 3.0) : powf(x, 1.0 / 3.0);
 }
-#endif
+#endif /* HAVE_CBRTF */
 
 #if !HAVE_COPYSIGN
 static av_always_inline double copysign(double x, double y)
@@ -71,12 +71,12 @@ static av_always_inline double copysign(double x, double y)
     uint64_t vy = av_double2int(y);
     return av_int2double((vx & UINT64_C(0x7fffffffffffffff)) | (vy & UINT64_C(0x8000000000000000)));
 }
-#endif
+#endif /* HAVE_COPYSIGN */
 
 #if !HAVE_COSF
 #undef cosf
 #define cosf(x) ((float)cos(x))
-#endif
+#endif /* HAVE_COSF */
 
 #if !HAVE_ERF
 static inline double ff_eval_poly(const double *coeff, int size, double x) {
@@ -276,16 +276,16 @@ static inline double erf(double z)
     else
         return 1;
 }
-#endif
+#endif /* HAVE_ERF */
 
 #if !HAVE_EXPF
 #undef expf
 #define expf(x) ((float)exp(x))
-#endif
+#endif /* HAVE_EXPF */
 
 #if !HAVE_EXP2
 #undef exp2
-#define exp2(x) exp((x) * 0.693147180559945)
+#define exp2(x) exp((x) * M_LN2)
 #endif /* HAVE_EXP2 */
 
 #if !HAVE_EXP2F
@@ -362,7 +362,6 @@ static av_always_inline av_const int avpriv_isnan(double x)
 #endif /* HAVE_ISNAN */
 
 #if !HAVE_HYPOT
-#undef hypot
 static inline av_const double hypot(double x, double y)
 {
     double ret, temp;
@@ -387,7 +386,7 @@ static inline av_const double hypot(double x, double y)
 #if !HAVE_LDEXPF
 #undef ldexpf
 #define ldexpf(x, exp) ((float)ldexp(x, exp))
-#endif
+#endif /* HAVE_LDEXPF */
 
 #if !HAVE_LLRINT
 #undef llrint
@@ -412,12 +411,12 @@ static inline av_const double hypot(double x, double y)
 #if !HAVE_LOG10F
 #undef log10f
 #define log10f(x) ((float)log10(x))
-#endif
+#endif /* HAVE_LOG10F */
 
 #if !HAVE_SINF
 #undef sinf
 #define sinf(x) ((float)sin(x))
-#endif
+#endif /* HAVE_SINF */
 
 #if !HAVE_RINT
 static inline double rint(double x)