configure.ac: disable CUDA-accelerated FFmpeg convs on macOS

Support for CUDA on macOS is just legacy and the stuff won't compile
(C++ interferes with lavc headers with the old compiler - tested with
CUDA 10.2 on macOS 10.13, which should be the last compatible).

configure.ac

@@ -1825,7 +1825,8 @@ if test $libavcodec_req != no && test $found_libavcodec = yes &&
 then
         LIBAVCODEC_COMMON="src/libavcodec/lavc_common.o src/libavcodec/utils.o"
         LIBAVCODEC_VIDEO="src/libavcodec/lavc_video.o src/libavcodec/from_lavc_vid_conv.o src/libavcodec/to_lavc_vid_conv.o"
-        if test "$lavc_cuda_conv_req" != no && test "$FOUND_CUDA" = yes; then
+        if test "$lavc_cuda_conv_req" != no && test "$FOUND_CUDA" = yes &&
+                        system != MacOSX; then
                 AC_DEFINE([HAVE_LAVC_CUDA_CONV], [1], [Build with lavc CUDA conversions])
                 to_lavc_cuda_obj=src/libavcodec/to_lavc_vid_conv_cuda.o
                 LIBAVCODEC_VIDEO="$LIBAVCODEC_VIDEO $to_lavc_cuda_obj src/libavcodec/from_lavc_vid_conv_cuda.o"