IS_ENABLED macro

Check for macro definition in compiler-visible expressions This trick was pioneered in Linux as the config_enabled() macro. The madness has the effect of taking a macro value that may be defined to "1" (e.g. CONFIG_MYFEATURE), or may not be defined at all and turning it into a literal expression that can be used at "runtime". That is, it works similarly to "defined(CONFIG_MYFEATURE)" does except that it is an expansion that can exist in a standard expression and be seen by the compiler and optimizer. Thus much ifdef usage can be replaced with cleaner expressions like: if (IS_ENABLED(CONFIG_MYFEATURE)) myfeature_enable(); INTERNAL First pass just to expand any existing macros, we need the macro value to be e.g. a literal "1" at expansion time in the next macro, not "(1)", etc... Standard recursive expansion does not work.