Disable DateTime::exactGmtNow() under MacOS

`clock_gettime(CLOCK_REALTIME, &t);` does not seem to
be available under MacOS

chrono/datetime.cpp

@@ -277,7 +277,7 @@ const char *DateTime::printDayOfWeek(DayOfWeek dayOfWeek, bool abbreviation)
     return "";
 }
 
-#if defined(PLATFORM_UNIX)
+#if defined(PLATFORM_UNIX) && !defined(PLATFORM_MAC)
 /*!
  * \brief Returns a DateTime object that is set to the current date and time on this computer, expressed as the GMT time.
  * \remarks

chrono/datetime.h

@@ -87,7 +87,7 @@ public:
     static constexpr DateTime unixEpochStart();
     static DateTime now();
     static DateTime gmtNow();
-#if defined(PLATFORM_UNIX)
+#if defined(PLATFORM_UNIX) && !defined(PLATFORM_MAC)
     static DateTime exactGmtNow();
 #endif
     constexpr static bool isLeapYear(int year);