incorrect handling of variadic macro calls with empty arguments

[mycoboco]

beluga's preprocessor goes wrong when the first argument to the variadic part is empty, that is, the first token for __VA_ARGS__ is a comma.

For example,

#define foo(...) bar(__VA_ARGS__, end)
#define bar(...) __VA_ARGS__

foo()

gives

t.c:1:35: ERROR - too many arguments to macro `bar'
  #define foo(...) bar(__VA_ARGS__, end)
                                    ^~~
t.c:4:1: note - expanded from here
  foo()
  ^~~~~
t.c:2:9: note - see this definition
  #define bar(...) __VA_ARGS__
          ^~~

[mycoboco]

In recarg() from mcr.c, if the first token to the variadic part is a comma, the comma is not merged into a single item because the accumulated number of arguments, n in the code, is less than the number of parameters, which makes the comma separate arguments.

[mycoboco]

Giving a comma to the variadic part as the first token cannot:

• trigger too many arguments to macro error;
• trigger C90 does not support empty argument to macro warning; and
• set *pnoarg.

But, handling for too many arguments to macro warning is still necessary.