Fix #368 (__VA_OPT__ is not handled good enough)

[glankk]

Fixes __VA_OPT__ by expanding it first, before further recursive expansions in variadic macros.

When a variadic macro containing a __VA_OPT__ token is parsed, the macro is split into a different token list for the two possible cases, where each occurence of __VA_OPT__ is either expanded and discarded. The correct token list is selected during macro expansion by examining the number of arguments, this way most of the extra processing required is only done once during the macro parsing, and only for variadic macros containing __VA_OPT__.