undef signal_set in folly/portability/Event.h

Summary:
In the sync from fbsource to ovrsource, we found that signal_set macro from libevent
has a conflict with boost::asio::signal_set function in boost_context.

e.g. failure https://www.internalfb.com/intern/sandcastle/job/18014398839678371/

This undef removes the build failures on the ovrsource side.

Reviewed By: yfeldblum

Differential Revision: D23941679

fbshipit-source-id: 25e208bdbc93f289d787ca742623fd6815863705

folly/portability/Event.h

@@ -25,9 +25,18 @@
 
 #ifdef _MSC_VER
 #include <event2/event_compat.h> // @manual
+// The signal_set macro from libevent 2 compat conflicts with the
+// boost::asio::signal_set function
+#undef signal_set
 #include <folly/portability/Fcntl.h>
 #endif
 
+// The signal_set macro from libevent 1.4.14b-stable conflicts with the
+// boost::asio::signal_set function
+#if _EVENT_NUMERIC_VERSION == 0x01040e00
+#undef signal_set
+#endif
+
 #include <folly/net/detail/SocketFileDescriptorMap.h>
 
 namespace folly {