lavu/libm: add copysign hack

For systems with broken libms.
Tested with NAN, -NAN, INFINITY, -INFINITY, +/-x for regular double x and
combinations of these.

Old versions of MSVC need some UINT64_C hackery.
Reviewed-by: Ronald S. Bultje <rsbultje@gmail.com>
Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

configure

@@ -2851,7 +2851,7 @@ cropdetect_filter_deps="gpl"
 delogo_filter_deps="gpl"
 deshake_filter_select="pixelutils"
 drawtext_filter_deps="libfreetype"
-dynaudnorm_filter_deps="copysign erf"
+dynaudnorm_filter_deps="erf"
 ebur128_filter_deps="gpl"
 eq_filter_deps="gpl"
 fftfilt_filter_deps="avcodec"

libavutil/libm.h

@@ -62,6 +62,15 @@ static av_always_inline float cbrtf(float x)
 }
 #endif
 
+#if !HAVE_COPYSIGN
+static av_always_inline double copysign(double x, double y)
+{
+    uint64_t vx = av_double2int(x);
+    uint64_t vy = av_double2int(y);
+    return av_int2double((vx & UINT64_C(0x7fffffffffffffff)) | (vy & UINT64_C(0x8000000000000000)));
+}
+#endif
+
 #if !HAVE_COSF
 #undef cosf
 #define cosf(x) ((float)cos(x))