Xcode: Add "outputPaths" to custom command script build phase

This is needed for Xcode 9's "New Build System", whose release notes
mention "that output must be declared as an explicit output by the
script which generates it" in reference to outputs of custom script
build phases.

Help/release/3.9.rst

@@ -34,6 +34,8 @@ Generators
   This is an experimental feature and can be activated by setting the
   :variable:`CMAKE_XCODE_GENERATE_SCHEME` variable to a ``TRUE`` value.
 
+* The :generator:`Xcode` generator now supports Xcode 9.
+
 Commands
 --------
 

Source/cmGlobalXCodeGenerator.cxx

@@ -1517,6 +1517,17 @@ void cmGlobalXCodeGenerator::AddCommandsToBuildPhase(
   makecmd += " all";
   buildphase->AddAttribute("shellScript", this->CreateString(makecmd));
   buildphase->AddAttribute("showEnvVarsInLog", this->CreateString("0"));
+
+  cmXCodeObject* outputFiles = this->CreateObject(cmXCodeObject::OBJECT_LIST);
+  for (std::vector<cmCustomCommand>::const_iterator i = commands.begin();
+       i != commands.end(); ++i) {
+    std::vector<std::string> const& outputs = i->GetOutputs();
+    for (std::vector<std::string>::const_iterator j = outputs.begin();
+         j != outputs.end(); ++j) {
+      outputFiles->AddObject(this->CreateString(*j));
+    }
+  }
+  buildphase->AddAttribute("outputPaths", outputFiles);
 }
 
 void cmGlobalXCodeGenerator::CreateCustomRulesMakefile(