folly: define FOLLY_SANITIZE_THREAD convenience macro for TSAN

Summary:
This defines the macro `FOLLY_SANITIZE_THREAD`, similar to `FOLLY_SANITIZE_ADDRESS`.

Note that gcc *just* landed support for the `__SANITIZE_THREAD__` macro two weeks
ago: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64354

Reviewed By: igorsugak

Differential Revision: D3389021

fbshipit-source-id: df6497a7741657c297ee812ed07cbae102b6810d

folly/CPortability.h

@@ -55,6 +55,15 @@
 # define FOLLY_DISABLE_ADDRESS_SANITIZER
 #endif
 
+/* Define a convenience macro to test when thread sanitizer is being used
+ * across the different compilers (e.g. clang, gcc) */
+#if defined(__clang__)
+# if __has_feature(thread_sanitizer)
+#  define FOLLY_SANITIZE_THREAD 1
+# endif
+#elif defined(__GNUC__) && __SANITIZE_THREAD__
+# define FOLLY_SANITIZE_THREAD 1
+#endif
 
 /**
  * ASAN/MSAN/TSAN define pre-processor symbols: