Compile time if statements

[Akuli]

I want:

if WINDOWS:
    declare GetModuleFileNameA(hModule: void*, lpFilename: byte*, nSize: int) -> int
elif MACOS:
    declare _NSGetExecutablePath(buf: byte*, bufsize: int*) -> int
else:
    declare readlink(linkpath: byte*, result: byte*, result_size: long) -> long

def find_current_executable() -> byte*:
    if WINDOWS:
        # use GetModuleFileNameA()
    elif MACOS:
        # use _NSGetExecutablePath() 
    else:
        # use readlink()

There is currently no good way to write platform-specific code, and I have ended up with some amusing workarounds:

https://github.com/Akuli/jou/blob/914200d20bfbcd17d660c18689ac2f910a94c2da/src/codegen.c#L189-L196

Previously mentioned in https://github.com/Akuli/jou/issues/383#issuecomment-1500832302