It's really a bug in flex that these functions are generated with neither
a declaration nor the 'static' keyword, but we can at least avoid the
warnings this way.
Previously, if the outer #ifdef/#ifndef evaluated to false, the inner
directive would not be parsed correctly, (the identifier as the subject
of the #ifdef/#ifndef would inadvertently be skipped along with the other
content correctly being skipped).
We fix this by setting the lexing_if state in each case here.
We also add a new test to the test suite to ensure that this case is tested.
