Set TARGET_OS_* from GN, falling back to HOST_OS_*.

runtime/platform/globals.h

Index: runtime/platform/globals.h
diff --git a/runtime/platform/globals.h b/runtime/platform/globals.h
index c504430fd09aeb7529b915b095fcb11b167e385e..21eb19a9b3e01f0ed82f759fb70c387af49e7ecc 100644
--- a/runtime/platform/globals.h
+++ b/runtime/platform/globals.h
@@ -102,9 +102,6 @@
 // the value defined in TargetConditionals.h
 #define HOST_OS_MACOS 1
 #if TARGET_OS_IPHONE
-// Test for this #define by saying '#if TARGET_OS_IOS' rather than the usual
-// '#if defined(TARGET_OS_IOS)'. TARGET_OS_IOS is defined to be 0 in
-// XCode >= 7.0. See Issue #24453.
 #define HOST_OS_IOS 1
 #endif
 
@@ -384,17 +381,45 @@ typedef simd128_value_t fpu_register_t;
 #endif
 
 
-#if defined(TARGET_ARCH_ARM)
-#if defined(TARGET_ABI_IOS) && defined(TARGET_ABI_EABI)
-#error Both TARGET_ABI_IOS and TARGET_ABI_EABI defined.
-#elif !defined(TARGET_ABI_IOS) && !defined(TARGET_ABI_EABI)
-#if defined(HOST_OS_MAC)
-#define TARGET_ABI_IOS 1
+#if !defined(TARGET_OS_ANDROID)

[zra, 2017/03/17 17:24:24]

Why not:
#if !defined(...) && \
     !defined(...) && \
     ...

[rmacnak, 2017/03/17 18:22:16]

Copied idiom from the equivalent section for TARGET_ARCH. Updated both to &&.

+#if !defined(TARGET_OS_FUCHSIA)
+#if !defined(TARGET_OS_IOS)
+#if !defined(TARGET_OS_LINUX)
+#if !defined(TARGET_OS_MACOS)
+#if !defined(TARGET_OS_WINDOWS)
+// No target OS specified; pick the one matching the host OS.
+#if defined(HOST_OS_ANDROID)
+#define TARGET_OS_ANDROID 1
+#elif defined(HOST_OS_FUCHSIA)
+#define TARGET_OS_FUCHSIA 1
+#elif defined(HOST_OS_IOS)
+#define TARGET_OS_MACOS 1
+// Test for this #define by saying '#if TARGET_OS_IOS' rather than the usual
+// '#if defined(TARGET_OS_IOS)'. TARGET_OS_IOS is defined to be 0 in
+// XCode >= 7.0. See Issue #24453.

[zra, 2017/03/17 17:24:24]

Can we use a different symbol? Maybe TARGET_OS_MACOS_IOS? TARGET_OS_IPHONE_OS?

[rmacnak, 2017/03/17 18:22:16]

Sure. TARGET_OS_MACOS_IOS seems like a good choice because it reflects our
treatment of iOS as a subtype of Mac in the platform-specific files.

+#define TARGET_OS_IOS 1
+#elif defined(HOST_OS_LINUX)
+#define TARGET_OS_LINUX 1
+#elif defined(HOST_OS_MACOS)
+#define TARGET_OS_MACOS 1
+#elif defined(HOST_OS_WINDOWS)
+#define TARGET_OS_WINDOWS 1
 #else
-#define TARGET_ABI_EABI 1
+#error Automatic target OS detection failed.
+#endif
 #endif
 #endif
-#endif  // TARGET_ARCH_ARM
+#endif
+#endif
+#endif
+#endif
+
+
+// TARGET_OS_IOS is defined as 0 on macOS. Always define it so we can test
+// uniformly.
+#if !defined(TARGET_OS_IOS)
+#define TARGET_OS_IOS 0
+#endif
 
 
 // Short form printf format specifiers