Add a pretty macro for deprecation

Summary: Make it easy for people to mark code as deprecated

Test Plan: Chose a random file and added the FOLLY_DEPRECATED macro to it

Reviewed By: yfeldblum@fb.com

Subscribers: ps, folly-diffs@, yfeldblum

FB internal diff: D1943248

Signature: t1:1943248:1427316689:b3305d134f374697e93d36bafbf0961eec1ef68a

folly/Portability.h

@@ -79,6 +79,13 @@ struct MaxAlign { char c; } __attribute__((__aligned__));
   __attribute__((__format__(__printf__, format_param, dots_param)))
 #endif
 
+// deprecated
+#if defined(__clang__) || defined(__GNUC__)
+# define FOLLY_DEPRECATED(msg) __attribute__((__deprecated__(msg)))
+#else
+# define FOLLY_DEPRECATED
+#endif
+
 // noreturn
 #if defined(_MSC_VER)
 # define FOLLY_NORETURN __declspec(noreturn)