Xcode: Add internal API to find xcodebuild

Teach the Xcode generator to compute the location of this tool or the
cmakexbuild wrapper.  Add internal APIs to get the locations on demand.

Use the "cmakexbuild" wrapper for Xcode < 4, and "xcodebuild" for modern
Xcode.

Source/cmGlobalXCodeGenerator.h

@@ -212,6 +212,11 @@ protected:
   std::vector<cmXCodeObject*> XCodeObjects;
   cmXCodeObject* RootObject;
 private:
+  std::string const& GetXcodeBuildCommand();
+  std::string FindXcodeBuildCommand();
+  std::string XcodeBuildCommand;
+  bool XcodeBuildCommandInitialized;
+
   void PrintCompilerAdvice(std::ostream&, std::string const&,
                            const char*) const {}