NetBSD build fix: A forgotten $extraflags caused -lm to fall off.

patch by Jan Knutar <jknutar -- at -- nic -- dot -- fi>

Originally committed as revision 4619 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -291,7 +291,7 @@ dv1394="no"
 make="gmake"
 LDFLAGS="$LDFLAGS -export-dynamic"
 case `uname -r` in
-2.*) extralibs="-lossaudio"
+2.*) extralibs="$extralibs -lossaudio"
 ;;
 esac
 ;;