Fix a Clang warning on OSX

In file included from unix_semaphore.cc:24:
./utility.hh:53:10: warning: private field 'm_pSemaphore' is not used
[-Wunused-private-field]
  sem_t *m_pSemaphore;
           ^
Merge pull request #1509 from rubenk/osx-fix-warning

Fix a Clang warning on OSX