implement variadic macros

[mycoboco]

This enables to test sea-canary against redis code.

[mycoboco]

Supporting variadic macros itself is easy, but due to the constraint from 6.10.3:

The identifier __VA_ARGS__ shall occur only in the replacement-list of a function-like macro that uses the ellipsis notation in the parameters.

issuing diagnostics against every appearance of __VA_ARGS__ is cumbersome.

[mycoboco]

Places to handle __VA_ARGS__:

• #define __VA_ARGS__
• #ifdef __VA_ARGS__
• #ifndef __VA_ARGS__
• #undef __VA_ARGS__
• #error ... __VA_ARGS__
• #pragma __VA_ARGS__

[mycoboco]

Diagnostics for unrecognized #pragmas seem to be enough.
__VA_ARGS__ needs to be detected only when it can appear in supported #pragmas.

[mycoboco]

To compare to gcc and clang:
gcc 5.4.0

• diagnoses uses of __VA_ARGS__
  • on non-directives,
  • in extra tokens,
  • on unrecognized #pragma and
  • on #error,
• and does not
  • in branch tests ignored by conditional inclusion.

clang 8.1.0

• issues uses of __VA_ARGS__
  • on non-directives,
  • in extra tokens and
  • on unrecognized #pragma and
  • in branch tests ignored by conditional inclusion.
• and does not
  • on #error

beluga

• issues uses of __VA_ARGS__
  • on #error,
• and does not
  • on non-directives,
  • in extra tokens,
  • on unrecognized #pragma and
  • in branch tests ignored by conditional inclusion.

Not issuing diagnostics for __VA_ARGS__ can be excused when other diagnostics are in place.