Add GCC extension

[Prokrust]

/Zx option support several GCC extension.
For the convenience of porting the code from GCC, clang it is worth adding support:
Statements and Declarations in Expressions - ({ ... })
Macros with a Variable Number of Arguments - Support  ## __VA_ARGS__

[Pelle]

Sure, but not trivial to implement. Also, since these are extensions they will always have lower priority than standard stuff. Maybe some day...