Xcode: Use EFFECTIVE_PLATFORM_NAME reference in ComputeOutputDir

Caveats apply: only valid when not overriding default value for output
dir *and* there is a CMAKE_XCODE_EFFECTIVE_PLATFORMS value.

For now, CMAKE_XCODE_EFFECTIVE_PLATFORMS must be defined in each
project, or in the cache for a build.

Code relying on value of LOCATION and similar properties may not
work if it depends on file existence on disk since CMake will not
evaluate $(EFFECTIVE_PLATFORM_NAME). This feature is only enabled
for making it easier to build Xcode iOS projects where the developer
wants to be able to switch easily between simulator and device builds.

Source/cmTarget.cxx

@@ -3757,8 +3757,12 @@ bool cmTarget::ComputeOutputDir(const char* config,
   // The generator may add the configuration's subdirectory.
   if(config && *config)
     {
+    const char *platforms = this->Makefile->GetDefinition(
+      "CMAKE_XCODE_EFFECTIVE_PLATFORMS");
+    std::string suffix =
+      usesDefaultOutputDir && platforms ? "$(EFFECTIVE_PLATFORM_NAME)" : "";
     this->Makefile->GetLocalGenerator()->GetGlobalGenerator()->
-      AppendDirectoryForConfig("/", config, "", out);
+      AppendDirectoryForConfig("/", config, suffix.c_str(), out);
     }
 
   return usesDefaultOutputDir;