[GCC-XML] Failure to handle __is_enum

[Neal Becker]

I have the current gcc_xml cvs.  It seems to choke on boost-1.37.0, where we 
use __is_enum if gcc is 4.3. 

Running on gccxml, it seems to identify itself as gcc4.3, but it doesn't seem 
to understand __is_enum builtin.

What's the story here?  I think gccxml is really gcc4.2, and doesn't support 
this builtin?  In that case, how do I run gccxml and tell it to use the gcc4.2 
config instead of gcc4.3?