These are supported by gcc and clang, and I don't think MSVC works well
with our code anyway, so let's keep things simple and use these
directly.

Clang complains loudly about this with Wformat=2, so add
a __printf_attr to help out.

Fixes:

/home/shade/dev/tigervnc/common/rfb/Logger.cxx:48:35: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
  vsnprintf(buf1, sizeof(buf1)-1, format, ap);
                                  ^~~~~~
1 error generated.

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@589 3789f03b-4d11-0410-bbf8-ca57d06f2519

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/branches/merge-with-vnc-4.1.1@522 3789f03b-4d11-0410-bbf8-ca57d06f2519

git-svn-id: svn://svn.code.sf.net/p/tigervnc/code/trunk@2 3789f03b-4d11-0410-bbf8-ca57d06f2519