terminating element in a varargs list). A sentinel needs to be NULL,
    not 0 - which doesn't make a difference on 32bit but matters on 64bit.
    Furthermore it can be told that functions have a printf-like format
    string and argument list so that they can verify that both match. To
    use these features certain attributes need to be set - which are
    compiler specific. To do this we define macros which are expanded
    depending on the compiler version. For now we put those in
    include/Xfuncproto.h (the XFree86 DDX layer contains a file compiler.h
    which however is not visible outside the DDX) (Bugzilla #3268).