Mac: Fix build with CMake 3.16+

CMake 3.16 now includes an OBJC language, so we need to enable that
language rather than fooling CMake into compiling the .m files by
labeling them as C code.

unix/vncviewer/CMakeLists.txt

@@ -10,10 +10,17 @@ execute_process(COMMAND chmod +x vncviewer
 include_directories(${JAVA_INCLUDE_PATH} ${JAVA_INCLUDE_PATH2}
 	${TJPEG_INCLUDE_DIR})
 
+if(CMAKE_VERSION VERSION_EQUAL "3.16" OR CMAKE_VERSION VERSION_GREATER "3.16")
+	enable_language(OBJC)
+endif()
+
 set(HELPER_SOURCES turbovnchelper.m JRSwizzle.m
 	${CMAKE_SOURCE_DIR}/common/turbojpeg-jni/turbojpeg-jni.c)
 add_library(turbovnchelper SHARED ${HELPER_SOURCES} usocket.c)
-set_source_files_properties(${HELPER_SOURCES} PROPERTIES LANGUAGE C)
+if(CMAKE_VERSION VERSION_LESS "3.16")
+	set_source_files_properties(${HELPER_SOURCES} PROPERTIES LANGUAGE C)
+endif()
+
 set_target_properties(turbovnchelper PROPERTIES
 	LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/java)
 target_link_libraries(turbovnchelper "-framework Cocoa" ${TJPEG_LIBRARY})