Error building on Mac OS X: error: use of undeclared identifier 'CLOCK_MONOTONIC_RAW'

On Mac OS 10.9.5

I've install all dependencies except eigen3 (since this isn't something explicitly installed to the OS).

Running make in ../parameter_server yields:

```
In file included from src/util/file.cc:1:
In file included from ./src/util/file.h:8:
In file included from ./src/util/common.h:42:
./src/util/resource_usage.h:35:17: error: use of undeclared identifier 'CLOCK_MONOTONIC_RAW'
clock_gettime(CLOCK_MONOTONIC_RAW, &tv);
^
./src/util/resource_usage.h:41:17: error: use of undeclared identifier 'CLOCK_MONOTONIC_RAW'
clock_gettime(CLOCK_MONOTONIC_RAW, &curr);
^
2 errors generated.
make: *** [build/util/file.o] Error 1
```

Appears related to clock_gettime not being present on Mac OS X:
http://stackoverflow.com/questions/5167269/clock-gettime-alternative-in-mac-os-x