semantics, allowing gcc and clang to check the format string against the
arguments. Enable its use on SkString (printf, appendf, and prependf). Also
define an SK_SIZE_T_SPECIFIER macro so there's a cross-platform way of
printing a size_t.
Review URL: http://codereview.appspot.com/6375043/
git-svn-id: http://skia.googlecode.com/svn/trunk@4485 2bbb7eff-a529-9590-31e7-b0007b416f81
