Include the OS X specific header to use the OS specific macros.

Hi,
The pull request I sent on Oct 29 in which I was supposed to fix compilation errors on OS X 10.9 (Mavericks) was not correct. This pull request fixes this issue.

The commit 2baa370 assumed that the macros for checking whether the user
compiles on OS X 10.9 are defined, but it was wrong because the header
which defines the macros was not included. This commit fixes this issue.