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.