Support GCC-specific __extension__ keyword

[alexdowad]

Some projects which are specifically written to be compiled by GCC use GCC-specific extensions to the C language, such as the __extension__ keyword. This is an interesting extension; it has no effect on the meaning of the code, but merely suppresses compiler warnings. Therefore, the AST does not need to change in any way.

The modifications to the grammar were made after carefully examining the source for the GCC parser to see where __extension__ can be used in GNU C.