mesa: prevent common string formatting security issues

mesa: prevent common string formatting security issues

Adds a compile-time error for obvious security issues like:

  printf(string_var);

The proposed flag is more tolerant than -Wformat-nonliteral.
Specifically, it tolerates common mesa formatting like:

  static const char *shader_template = "really long string %d";
  printf(shader_template, uniform_number);

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110833
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>