WG14 adopted N2645 and WG21 EWG has accepted P2334 in principle (still subject to full EWG vote + CWG review + plenary vote), which add support for #elifdef as shorthand for #elif defined and #elifndef as shorthand for #elif !defined. This patch adds support for the new preprocessor directives.
In this version of the patch, I am supporting the feature in all C and C++ modes. This seems like more useful functionality than diagnosing an unknown preprocessor directive in older language modes, and is a conforming extension in those modes anyway as use of an unknown directive is undefined behavior. However, it was not clear to me whether we want to issue a pedantic warning for this or not or whether we felt it was important to document this behavior explicitly.