Add support for Mac OS X 10.10 (Yosemite)

* The compat file is no longer needed but removing it for Yosemite and
  keeping for other versions of Mac OS X is cumbersome. Instead, using
  the C preprocessor to remove the function declarations.
* The readlinkat code conflicts with 10.10 system headers (int instead
  of size_t). Since it works as-is on other OS X versions, the same
  preprocessor macro is used to remove it in 10.10.

I am not sure why the other .c files for *at functions do not need to be changed for Yosemite support.