Refactored threadlocal logic.

Refactored the threadlocal logic for Android and IOS into logic in
platform_macro.h which computes a GOOGLE_PROTOBUF_NO_THREADLOCAL
define which is then used elsewhere.  This allows new platforms
without THREADLOCAL to be easily defined.

src/google/protobuf/arena.cc

diff --git a/src/google/protobuf/arena.cc b/src/google/protobuf/arena.cc
index 9600964..ed1c5ef 100755
--- a/src/google/protobuf/arena.cc
+++ b/src/google/protobuf/arena.cc
@@ -43,7 +43,7 @@
   static GOOGLE_THREAD_LOCAL ThreadCache thread_cache_ = { -1, NULL };
   return thread_cache_;
 }
-#elif defined(GOOGLE_PROTOBUF_OS_ANDROID) || defined(GOOGLE_PROTOBUF_OS_IPHONE)
+#elif defined(GOOGLE_PROTOBUF_NO_THREADLOCAL)
 Arena::ThreadCache& Arena::thread_cache() {
   static internal::ThreadLocalStorage<ThreadCache>* thread_cache_ =
       new internal::ThreadLocalStorage<ThreadCache>();

src/google/protobuf/arena.h

diff --git a/src/google/protobuf/arena.h b/src/google/protobuf/arena.h
index 6c3f606..51149ba 100644
--- a/src/google/protobuf/arena.h
+++ b/src/google/protobuf/arena.h
@@ -39,6 +39,7 @@
 #include <google/protobuf/stubs/atomic_sequence_num.h>
 #include <google/protobuf/stubs/atomicops.h>
 #include <google/protobuf/stubs/common.h>
+#include <google/protobuf/stubs/platform_macros.h>
 #include <google/protobuf/stubs/type_traits.h>
 
 namespace google {
@@ -528,7 +529,7 @@
   // Thread local variables cannot be exposed through DLL interface but we can
   // wrap them in static functions.
   static ThreadCache& thread_cache();
-#elif defined(GOOGLE_PROTOBUF_OS_ANDROID) || defined(GOOGLE_PROTOBUF_OS_IPHONE)
+#elif defined(GOOGLE_PROTOBUF_NO_THREADLOCAL)
   // Android ndk does not support GOOGLE_THREAD_LOCAL keyword so we use a custom thread
   // local storage class we implemented.
   // iOS also does not support the GOOGLE_THREAD_LOCAL keyword.

src/google/protobuf/stubs/atomicops.h

diff --git a/src/google/protobuf/stubs/atomicops.h b/src/google/protobuf/stubs/atomicops.h
index bbe2d84..cd20caa 100644
--- a/src/google/protobuf/stubs/atomicops.h
+++ b/src/google/protobuf/stubs/atomicops.h
@@ -56,6 +56,7 @@
 // Don't include this file for people not concerned about thread safety.
 #ifndef GOOGLE_PROTOBUF_NO_THREAD_SAFETY
 
+#include <google/protobuf/stubs/common.h>
 #include <google/protobuf/stubs/platform_macros.h>
 
 namespace google {

src/google/protobuf/stubs/common.h

diff --git a/src/google/protobuf/stubs/common.h b/src/google/protobuf/stubs/common.h
index e4c35a2..49d1602 100644
--- a/src/google/protobuf/stubs/common.h
+++ b/src/google/protobuf/stubs/common.h
@@ -62,6 +62,8 @@
 #include <exception>
 #endif
 
+#include <google/protobuf/stubs/platform_macros.h>
+
 #if defined(__APPLE__)
 #include <TargetConditionals.h>  // for TARGET_OS_IPHONE
 #endif
@@ -1174,11 +1176,7 @@
   GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(MutexLockMaybe);
 };
 
-#if defined(__ANDROID__) || defined(GOOGLE_PROTOBUF_OS_ANDROID) || (defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE) || defined(GOOGLE_PROTOBUF_OS_IPHONE)
-// Android ndk does not support the __thread keyword very well yet. Here
-// we use pthread_key_create()/pthread_getspecific()/... methods for
-// TLS support on android.
-// iOS also does not support the __thread keyword.
+#if defined(GOOGLE_PROTOBUF_NO_THREADLOCAL)
 template<typename T>
 class ThreadLocalStorage {
  public:

src/google/protobuf/stubs/platform_macros.h

diff --git a/src/google/protobuf/stubs/platform_macros.h b/src/google/protobuf/stubs/platform_macros.h
index 2ce7fc8..9e0344d 100644
--- a/src/google/protobuf/stubs/platform_macros.h
+++ b/src/google/protobuf/stubs/platform_macros.h
@@ -31,8 +31,6 @@
 #ifndef GOOGLE_PROTOBUF_PLATFORM_MACROS_H_
 #define GOOGLE_PROTOBUF_PLATFORM_MACROS_H_
 
-#include <google/protobuf/stubs/common.h>
-
 #define GOOGLE_PROTOBUF_PLATFORM_ERROR \
 #error "Host platform was not detected as supported by protobuf"
 
@@ -111,4 +109,12 @@
 
 #undef GOOGLE_PROTOBUF_PLATFORM_ERROR
 
+#if defined(GOOGLE_PROTOBUF_OS_ANDROID) || defined(GOOGLE_PROTOBUF_OS_IPHONE)
+// Android ndk does not support the __thread keyword very well yet. Here
+// we use pthread_key_create()/pthread_getspecific()/... methods for
+// TLS support on android.
+// iOS also does not support the __thread keyword.
+#define GOOGLE_PROTOBUF_NO_THREADLOCAL
+#endif
+
 #endif  // GOOGLE_PROTOBUF_PLATFORM_MACROS_H_