Xcode: Select make program at build time

Extend the change made in commit v3.0.0-rc1~260^2~16 (Teach
GenerateBuildCommand to find its own make program, 2013-11-13) to have
the Xcode generator pick between "xcodebuild" and CMake's own copy of
"cmakexbuild" at build time based on the version of Xcode.

Source/cmGlobalXCodeGenerator.cxx

@@ -303,7 +303,7 @@ cmGlobalXCodeGenerator::GenerateBuildCommand(
 {
   // now build the test
   makeCommand.push_back(
-    this->SelectMakeProgram(makeProgram, "xcodebuild")
+    this->SelectMakeProgram(makeProgram, this->GetXcodeBuildCommand())
     );
 
   makeCommand.push_back("-project");