Comment 1 for bug 293807

Confirming, __builtin_va_arg_pack is undefined. The file /usr/share/gccxml-0.9/GCC/4.3/gccxml_builtins.h says:

 In order to really support these we would have to update GCC-XML's
 internal parser to be GCC 4.3. Since this has not yet been done we
 get most of the way by defining some of these as macros with fake
 answers. It is enough to get through GCC 4.3's C++98 standard
 library.

As workaround, you can add this at the end of the file:

 int __dummy__(...);
 #define __builtin_va_arg_pack __dummy__

The example you give "compiles" fine, and some quite complex code does as well.