change detection of apple OS's

also undef RMW_THREAD_LOCAL to cause compiler
error in the case that pthread must be used

Author: wjwwood

rmw/include/rmw/macros.h

@@ -15,22 +15,36 @@
 #ifndef RMW__MACROS_H_
 #define RMW__MACROS_H_
 
+// This block either sets RMW_THREAD_LOCAL or RMW_THREAD_LOCAL_PTHREAD.
 #if defined _WIN32 || defined __CYGWIN__
+// Windows or Cygwin
   #define RMW_THREAD_LOCAL __declspec(thread)
 #elif defined __APPLE__
-  #include <Availability.h>
-  #if defined(__IPHONE_OS_VERSION_MIN_REQUIRED)
-    // iOS 10 added support for thread local storage
-    #if __IPHONE_OS_VERSION_MAX_ALLOWED < 100000
-      #define RMW_THREAD_LOCAL_PTHREAD 1
-      #define RMW_THREAD_LOCAL
+// Apple OS's
+  #include <TargetConditionals.h>
+  #if TARGET_IPHONE_SIMULATOR || TARGET_OS_IPHONE
+// iOS Simulator or iOS device
+    #include <Availability.h>
+    #if defined(__IPHONE_OS_VERSION_MIN_REQUIRED)
+      #if __IPHONE_OS_VERSION_MAX_ALLOWED >= 100000
+// iOS >= 10, thread local storage was added in iOS 10
+        #define RMW_THREAD_LOCAL _Thread_local
+      #else
+// iOS < 10, no thread local storage, so use pthread instead
+        #define RMW_THREAD_LOCAL_PTHREAD 1
+        #undef RMW_THREAD_LOCAL
+      #endif
+    #else
+      #error "Unknown iOS version"
     #endif
-  #endif
-
-  #ifndef RMW_THREAD_LOCAL_PTHREAD
+  #elif TARGET_OS_MAC
+// macOS
     #define RMW_THREAD_LOCAL _Thread_local
+  #else
+    #error "Unknown Apple platform"
   #endif
 #else
+// Some other non-Windows, non-cygwin, non-apple OS
   #define RMW_THREAD_LOCAL _Thread_local
 #endif