When applying the changes in 8edd3464af,
it seems that this bit got merged incorrectly and no test coverage
caught the issue. This fixes the diagnostic and adds a test.
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.