POSIX: Remove unnecessary feature test macro.

This is a no-op since the line was already commented out, to make it
compile on FreeBSD. But it deserves an explanation:

The _POSIX_C_SOURCE macro is meant to restrict the C library to a
certain POSIX standard, not to enable features. This is inappropriate
here because:

1. Jack already uses non-POSIX functions. E.g. JackPosixSemaphore.cpp
   references gettimeofday(), which is part of the X/Open specs.

2. C++ does not define a POSIX level to be based on. Headers from the
   C++ standard library may (and do) break when the POSIX feature level
   is restricted.

The canonical way is to only set the feature level through the
"--std=gnu++11" compile options.

posix/JackPosixSemaphore.cpp

@@ -17,8 +17,6 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 */
 
-// #define _POSIX_C_SOURCE 200112L
-
 #include "JackPosixSemaphore.h"
 #include "JackTools.h"
 #include "JackConstants.h"