CMake: might want to support memory checks using `leaks` on macOS

[agarny]

I really want to be able to test for memory leaks on macOS and valgrind/memcheck is "only" available on Linux. However, on macOS, there is the leaks program that can easily be used to check for memory leaks. All you need is to call it like so: leaks -atExit -- path/to/test.