GDB CVS won't build on OSF4.0's cc

[Alexandre Oliva]

Another problem showed up while building gdbtk.  gdb/defs.h #includes
<stdarg.h>, but tcl/generic/tclInt.h #includes <varargs.h>, because
HAS_STDARG is not defined.

It turns out that the va_start macro defined in varargs.h ends up
overriding that of stdarg.h, so we call va_start with the wrong number
of arguments.

Since GDB assumes the existence of stdarg.h, I suppose this patch
would be acceptable.  Ok to install?