Fix build in OS X 10.9

This fixes the following build errors for Mavericks:
https://gist.github.com/federicobond/fe75ae7a418d072186cb

Compiler version:
````
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.1.0
Thread model: posix
````