#if constexpr statements [#else statements] [#endif]
constexpr - constant integer expression. Also can be global variable of type int, user-defined function of type int (function is executed while compiling; it runs in QM main thread), or expression with operators (expression is evaluated from left to right, regardless of operator priority).
statements - any statements.
If value of constexpr is not 0, are compiled statements after #if and skipped statements after #else. Otherwise, are skipped statements after #if and compiled statements after #else.
#if... cannot be in other #if... or #else block. If #endif is omitted, are compiled (or skipped) all following statements until end of text.
Unlike if, compiler directives are evaluated at compile time.
See also: predefined variables, predefined constants, make exe.
#if (_winnt>=6) ... #else ... #endif