Conditional_Compilation

 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.