Re-Gate setenv/unsetenv

Summary: `setenv`/`unsetenv` is not always declared the same way on all of the places folly is compiled.

Reviewed By: matbd

Differential Revision: D4462056

fbshipit-source-id: 6dd68f6fc5a6a435f145cbaa43bee02b5fa7170c

folly/portability/Environment.h

@@ -21,7 +21,8 @@
 extern "C" {
 #ifndef _WIN32
 extern char** environ;
-#endif
+#else
 int setenv(const char* name, const char* value, int overwrite);
 int unsetenv(const char* name);
+#endif
 }