Fixes to compile and link on OS X

Building master on OS X I hit the below errors which are both fixed by this pull request:

```sh
main.cpp:468:3: error: use of undeclared identifier 'sigemptyset'
  sigemptyset(&sa.sa_mask);
  ^
```

And

```sh
  CXXLD    ../map
Undefined symbols for architecture x86_64:
  "boost::system::system_category()", referenced from:
      boost::mutex::unlock() in backend.o
      boost::unique_lock<boost::mutex>::lock() in backend.o
      boost::mutex::lock() in backend.o
      boost::mutex::mutex() in backend.o
      __GLOBAL__I_a in backend.o
  "boost::system::generic_category()", referenced from:
      __GLOBAL__I_a in backend.o
ld: symbol(s) not found for architecture x86_64
```