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

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.
Merge pull request #3 from tetsuok/fix-osx-mavericks-build

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