Support elifdef and elifndef directives

[jefftrull]

Passed today in the C committee, as I understand it. Not sure about the year it will come out.

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2645.pdf

[jefftrull]

Destined for C23/C++23, I'm told.

[hkaiser]

Destined for C23/C++23, I'm told.

This shouldn't be hard to implement...