cpp.info: Else

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Else

   The `#else' directive can be added to a conditional to provide
alternative text to be used if the condition fails.  This is what it
looks like:
     #if EXPRESSION
     TEXT-IF-TRUE
     #else /* Not EXPRESSION */
     TEXT-IF-FALSE
     #endif /* Not EXPRESSION */
If EXPRESSION is nonzero, the TEXT-IF-TRUE is included and the
TEXT-IF-FALSE is skipped.  If EXPRESSION is zero, the opposite happens.
   You can use `#else' with `#ifdef' and `#ifndef', too.