Fix clang sysroot flag

Older versions accepted both with and without an '=', but newer versions
require the '=' to be omitted

Originally committed as revision 22274 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1862,8 +1862,8 @@ if test -n "$sysroot"; then
             add_ldflags  --sysroot="$sysroot"
         ;;
         clang)
-            add_cppflags -isysroot="$sysroot"
-            add_ldflags -isysroot="$sysroot"
+            add_cppflags -isysroot "$sysroot"
+            add_ldflags -isysroot "$sysroot"
         ;;
     esac
 fi