Provide a fallback version of the libm function trunc

This fixes compilation on DOS.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1119,6 +1119,7 @@ HAVE_LIST="
     sys_videoio_h
     ten_operands
     threads
+    trunc
     truncf
     vfp_args
     VirtualAlloc
@@ -2852,6 +2853,7 @@ check_mathfunc lrint
 check_mathfunc lrintf
 check_mathfunc round
 check_mathfunc roundf
+check_mathfunc trunc
 check_mathfunc truncf
 
 # these are off by default, so fail if requested and not available

libavutil/libm.h

@@ -86,6 +86,13 @@ static av_always_inline av_const float roundf(float x)
 }
 #endif /* HAVE_ROUNDF */
 
+#if !HAVE_TRUNC
+static av_always_inline av_const double trunc(double x)
+{
+    return (x > 0) ? floor(x) : ceil(x);
+}
+#endif /* HAVE_TRUNC */
+
 #if !HAVE_TRUNCF
 static av_always_inline av_const float truncf(float x)
 {