va_list function portability doco

[Kevin Ryde]

I don't suppose there's anything in the below which isn't a straight
consequence of the relevant standards, but perhaps a paragraph on the
traps might keep people out of trouble.  The call-by-reference
behaviour on powerpc for instance seems to me a particular trap if one
hasn't encountered it before.

        * autoconf.texi (Function Portability): Notes on va_list.

valist.texi
Description: TeXInfo document