Conditional compilation is provided by the following directives:

  #if constant-expression
     Checks whether the constant expression is nonzero (true).

  #ifdef identifier
     Checks whether the identifier is defined.

  #ifndef identifier
     Checks whether the identifier is undefined.

  #else
     Introduces source lines to be compiled as an alternative to the
     conditions tested by the previous directives.

  #elif constant-expression
     Delimits alternative source lines to be compiled if the constant
     expression in the corresponding #if, #ifdef, or #ifndef
     directive is false and if the additional constant expression
     presented in the #elif directive is true.  An #elif directive is
     optional.

  #endif
     Ends the scope of the previous directives.

  If the condition checked by #if, #ifdef, or #ifndef is true, then
  all lines between the #else, #elif, and #endif are ignored.  If the
  condition is false, then any lines between the conditional
  directive and the #else or #elif (if any) are ignored.  If there is
  no #else, then the lines between the conditional and the #endif are
  ignored.