C/pre-processor: detect if a __builtin function is available

Is it possible to somehow determine whether an intrinsic function, such as `__builtin_bswap16` is provided by the compiler? Preferably, I would like to be able to determine whether this function exists using just preprocessor. In my particular case, I was using `__builtin_bswap16` / 32 / 64 functions in my code which worked fine with GCC 4.x when compiling for 32-bit. Later I switched to a 64-bit Linux and noticed that `__builtin_bswap16` suddenly disappeared - I received a linker error: "undefined reference to `__builtin_bswap16'". I guess this has something to do with the availability of certain ASM operations in 64-bit mode. On a later occasion I was trying to compile this code on a different machine where unfortunately only an older version of GCC is installed and does not support these functions at all. I would like to make this code compilable everywhere, using `__builtin_bswap` functions if provided, and fall back to hand-coded `byteswap` routine if not. Is it possible to achieve this somehow with just preprocessor? My obvious attempt, e.g.: ``` ... #define MYBSWAP16(v) (v>>8)|(v<<8) #ifdef __builtin_bswap16 printf("bswap16 is defined : %04x\n", __builtin_bswap16(0x1234)); #else printf("bswap16 is not defined : %04x\n", MYBSWAP16(0x1234) ); #endif ... ``` was not successful, as `__builtin_bswap16`/32/64 are always evaluated to be undefined. Is there any way to make it work automatically within the C source, or is the only way to manually define constants in the Makefile, e.g. `HAVE_BSWAP` and pass them via `-D` option? Please note that my question is not necessarily specific to `__builtin_bswap`, I'm looking for a general way to detect if the certain functions are available.