Implement a more reliable detection scheme for os_unfair_lock.

The core issue here is the weak linking of the symbol, and in certain
environments--for instance, using the latest Xcode (8.1) with the latest
SDK (10.12)--os_unfair_lock may resolve even though you're compiling on
a host that doesn't support it (10.11).

We can use the availability macros to circumvent this problem, and
detect that we're not compiling for a target that is going to support
them and error out at compile time.  The other alternative is to do a
runtime check, but that presents issues for cross-compiling.

Author: jszakmeister

configure.ac

@@ -1708,10 +1708,15 @@ dnl Check for os_unfair_lock operations as provided on Darwin.
 
 JE_COMPILABLE([Darwin os_unfair_lock_*()], [
 #include <os/lock.h>
+#include <AvailabilityMacros.h>
 ], [
+	#if MAC_OS_X_VERSION_MIN_REQUIRED < 101200
+	#error "os_unfair_lock is not supported"
+	#else
 	os_unfair_lock lock = OS_UNFAIR_LOCK_INIT;
 	os_unfair_lock_lock(&lock);
 	os_unfair_lock_unlock(&lock);
+	#endif
 ], [je_cv_os_unfair_lock])
 if test "x${je_cv_os_unfair_lock}" = "xyes" ; then
   AC_DEFINE([JEMALLOC_OS_UNFAIR_LOCK], [ ])