Install MinGW shared libraries (dlls) to bindir

Originally committed as revision 8953 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1151,6 +1151,7 @@ case $targetos in
     ;;
   mingw32*)
     targetos=mingw32
+    shlibdir="$bindir"
     if enabled_all shared static; then
         cat <<EOF
 You can only build one library type at once on MinGW.