Add an exception for building on OSX.

Summary: The Homebrew setup fails to compile on OSX.

Reviewed By: @paulbiss

Differential Revision: D2212084

folly/String.cpp

@@ -342,7 +342,7 @@ fbstring errnoStr(int err) {
   } else {
     result.assign(buf);
   }
-#elif defined(HAVE_XSI_STRERROR_R)
+#elif defined(HAVE_XSI_STRERROR_R) || defined(__APPLE__) || defined(__ANDROID__)
   // Using XSI-compatible strerror_r
   int r = strerror_r(err, buf, sizeof(buf));