Use getentropy on macOS 10.12 and later.

Bug: 287
Change-Id: I40760bdba8dcaab9c5c38d52d6479138f52eccbd
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/37284
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/include/openssl/base.h b/include/openssl/base.h
index cb1affa..e347c09 100644
--- a/include/openssl/base.h
+++ b/include/openssl/base.h
@@ -124,6 +124,11 @@
 
 #if defined(__APPLE__)
 #define OPENSSL_APPLE
+// Note |TARGET_OS_MAC| is set for all Apple OS variants. |TARGET_OS_OSX|
+// targets macOS specifically.
+#if defined(TARGET_OS_OSX) && TARGET_OS_OSX
+#define OPENSSL_MACOS
+#endif
 #if defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE
 #define OPENSSL_IOS
 #endif