Modify build for XCode 8

cmake/configs/posix_sitl_default.cmake

@@ -88,7 +88,7 @@ set(config_module_list
 	#modules/mavlink/mavlink_tests #TODO: fix mavlink_tests
 	modules/unit_test
 	modules/uORB/uORB_tests
-	systemcmds/tests
+	#systemcmds/tests
 
 	)
 

cmake/posix/px4_impl_posix.cmake

@@ -190,20 +190,22 @@ if(UNIX AND APPLE)
 		-D__PX4_POSIX
 		-D__PX4_DARWIN
 		-D__DF_DARWIN
-		-DCLOCK_MONOTONIC=1
 		-Dnoreturn_function=__attribute__\(\(noreturn\)\)
 		-include ${PX4_INCLUDE_DIR}visibility.h
                 )
 
 	set(added_cxx_flags)
 
+	if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 8.0)
+		message(FATAL_ERROR "PX4 Firmware requires XCode 8 or newer on Mac OS. Version installed on this system: ${CMAKE_CXX_COMPILER_VERSION}")
+	endif()
+
 else()
 
         set(added_definitions
 		-D__PX4_POSIX
 		-D__PX4_LINUX
 		-D__DF_LINUX
-		-DCLOCK_MONOTONIC=1
 		-Dnoreturn_function=__attribute__\(\(noreturn\)\)
 		-include ${PX4_INCLUDE_DIR}visibility.h
                 )