Replace CMAKE_XCODE_EFFECTIVE_PLATFORMS with call to PlatformIsAppleIos

Currently the CMAKE_XCODE_EFFECTIVE_PLATFORMS property acts only as
a kind of toggle switch to enable iOS project layout features.
But instead of relying on this undocumented property, better detect
the presence of an iOS SDK directly.

Source/cmTarget.cxx

@@ -3566,10 +3566,9 @@ bool cmTarget::ComputeOutputDir(const std::string& config,
   // The generator may add the configuration's subdirectory.
   if(!conf.empty())
     {
-    const char *platforms = this->Makefile->GetDefinition(
-      "CMAKE_XCODE_EFFECTIVE_PLATFORMS");
+    bool iosPlatform = this->Makefile->PlatformIsAppleIos();
     std::string suffix =
-      usesDefaultOutputDir && platforms ? "$(EFFECTIVE_PLATFORM_NAME)" : "";
+      usesDefaultOutputDir && iosPlatform ? "$(EFFECTIVE_PLATFORM_NAME)" : "";
     this->Makefile->GetGlobalGenerator()->
       AppendDirectoryForConfig("/", conf, suffix, out);
     }

Tests/RunCMake/XcodeProject/XcodeBundles.cmake

@@ -6,7 +6,6 @@ enable_language(C)
 if(TEST_IOS)
   set(CMAKE_OSX_SYSROOT iphoneos)
   set(CMAKE_OSX_ARCHITECTURES "armv7")
-  set(CMAKE_XCODE_EFFECTIVE_PLATFORMS "-iphoneos;-iphonesimulator")
   set(CMAKE_XCODE_ATTRIBUTE_CODE_SIGNING_REQUIRED "NO")
   set(CMAKE_XCODE_ATTRIBUTE_ENABLE_BITCODE "NO")
 endif(TEST_IOS)

Tests/iOSNavApp/CMakeLists.txt

@@ -3,7 +3,6 @@ project(NavApp3)
 
 set(CMAKE_OSX_SYSROOT iphoneos4.3)
 set(CMAKE_OSX_ARCHITECTURES "armv6;armv7;i386")
-set(CMAKE_XCODE_EFFECTIVE_PLATFORMS "-iphoneos;-iphonesimulator")
 
 include_directories(
   ${CMAKE_CURRENT_SOURCE_DIR}