darwin: Fix build

[kirelagin]

Part of #13.

This fixes the build on macOS (and, hopefully, does not break it on Linux).

The thing is still extremely far from being usable on macOS, as it relies on the linux-ptrace package, which (as its name implies :) is very Linux specific.

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This change is 

[kirelagin]

(I should probably add that I actually did not test this change on Linux and, in particular, I am not sure that the CPP thing actually works as expected and adds SIGPOLL to the map on Linux.)

[qrilka]

@kirelagin I didn't look into the code but your branch looks ok on Linux, on my desktop Gentoo stack test finishing with

27 examples, 0 failures, 4 pending                                                                                                      
hatrace-0.1.0.0: Test suite hatrace-test passed
Completed 2 action(s).

[nh2]

I can confirm this fixes the compilation on Darwin. InvalidArgment in the tests, need to do that next :)

[kirelagin]

The problem is that linux-ptrace uses Linux-specific ptrace functionality and it makes calls with magic constants, rather than by including an OS header file, so it doesn’t fail to compile on other platforms, when it actually should. I am not even sure that all required functionality is supported on darwin, although I haven’t looked at what calls it makes and why in detail yet. My feeling is that the next step is creating darwin-ptrace and making it API-compatible.

[nh2]

Perhaps we should also just make our own ptrace module as part of hatrace, build that from scratch via header file constants, and when we're done and it works well, we can offer it back up to linux-ptrace.