Fix conflicts with new declaration with 'C' linkage

paddle/platform/environment.h

@@ -15,13 +15,12 @@ limitations under the License. */
 #pragma once
 
 #include <stdlib.h>
+#include <unistd.h>
 #include <vector>
 
 #include "paddle/platform/enforce.h"
 #include "paddle/string/piece.h"
 
-extern char** environ;
-
 namespace paddle {
 namespace platform {
 
@@ -46,6 +45,7 @@ inline std::string GetEnvValue(const std::string& name) {
 }
 
 inline std::vector<std::string> GetAllEnvVariables() {
+  extern char** environ;
   std::vector<std::string> vars;
   for (auto var = environ; *var != nullptr; ++var) {
     auto tail = string::Index(*var, "=");