use folly_struct_timezone to avoid name collision on win32 for polyfill

Summary:
On some WIN32 platforms #defines `timezone` as such we get a compilation
error due to name collision.

To fix let's use a folly_ prefix on our polyfill.

Reviewed By: yfeldblum

Differential Revision: D27220643

fbshipit-source-id: ccd4fa971388d53b8c5e8fba9f217776573ea370

folly/portability/SysTime.cpp

@@ -21,7 +21,7 @@
 #include <cstdint>
 
 extern "C" {
-int gettimeofday(timeval* tv, struct timezone*) {
+int gettimeofday(timeval* tv, folly_port_struct_timezone*) {
   constexpr auto posixWinFtOffset = 116444736000000000ULL;
 
   if (tv) {

folly/portability/SysTime.h

@@ -17,26 +17,34 @@
 #pragma once
 
 #ifndef _WIN32
+
 #include <sys/time.h>
+
+// win32 #defines timezone; this avoids collision
+using folly_port_struct_timezone = struct timezone;
+
 #else
+
 // Someone decided this was a good place to define timeval.....
 #include <folly/portability/Windows.h>
-struct timezone {
+
+struct folly_port_struct_timezone_ {
   int tz_minuteswest;
   int tz_dsttime;
 };
+using folly_port_struct_timezone = struct folly_port_struct_timezone_;
 
 extern "C" {
-// Note that this needs to explicitly be `struct timezone` due to the fact that
-// the python 3 headers `#define timezone _timezone` on Windows. `_timezone` is
-// a global field that contains information on the current timezone. By
-// explicitly specifying that this is a `struct`, we ensure that it's treated as
-// a type, regardless of what name that type actually is :)
-// Note that this will break if `gettimeofday` ever becomes declared as anything
-// other than `extern "C"`, as the mangled name would be dependent on whether
-// python had been included before this header.
-int gettimeofday(timeval* tv, struct timezone*);
+
+// We use folly_port_struct_timezone due to issues with #defines on Windows
+// platforms.
+// The python 3 headers `#define timezone _timezone` on Windows. `_timezone` is
+// a global field that contains information on the current timezone.
+// As such "timezone" is not a good name to use inside of C/C++ code on
+// Windows.  Instead users should use folly_port_struct_timezone instead.
+int gettimeofday(timeval* tv, folly_port_struct_timezone*);
 void timeradd(timeval* a, timeval* b, timeval* res);
 void timersub(timeval* a, timeval* b, timeval* res);
 }
+
 #endif