Give more verbose message when SOURCE_FILES variable is found not set.

Minor documentation update to stress that most of the build options supported by Urho3D project can also be pased to build script for configuring external project.
Add initial documentation on 'scaffolding' rake task.

Docs/GettingStarted.dox

@@ -344,7 +344,7 @@ PROJECT_ROOT/
  ┕ *.bat or *.sh
 \endcode
 
-The Source/ directory is where your project CMakeLists.txt and all your source codes should reside. The Bin/ directory is where all the executables output will be generated. In order to run the executable in this directory, you need to copy (or symlink) the CoreData/ and Data/ directories from Urho3D project Bin/ directory (or from Urho3D SDK installation) to here. You may also want to copy (or symlink) the batch files or shell script files from Urho3D project root directory (or from Urho3D SDK installation) to your project root directory. The Build/ directory will be created automatically by the Urho3D CMake module that will be discussed shortly below. The *.bat or *.sh scripts work together with the module to create a so-called out-of-source build tree to keep the Source/ directory clean from intermediate build files.
+The Source/ directory is where your project CMakeLists.txt and all your source codes should reside. The Bin/ directory is where all the executables output will be generated. In order to run the executable in this directory, you need to copy (or symlink) the CoreData/ and Data/ directories from Urho3D project Bin/ directory (or from Urho3D SDK installation) to here. You may also want to copy (or symlink) the batch files or shell script files from Urho3D project root directory (or from Urho3D SDK installation) to your project root directory. The Build/ directory will be created automatically by the Urho3D CMake module that will be discussed shortly below. The *.bat or *.sh scripts work together with the module to create a so-called out-of-source build tree to keep the Source/ directory clean from intermediate build files. Note that when you invoke one the *.bat or *.sh script, you can pass most of the \ref Build_Options "build options" supported by Urho3D project. In fact, you probably have to pass the similar build options that you use when building the Urho3D library to avoid any conflict.
 
 \section UsingLibraryFromProjectRootTree From Urho3D project root tree
 
@@ -445,6 +445,17 @@ export PKG_CONFIG_PATH=${RASPI_ROOT}/usr/local/lib/pkgconfig
 $CC -o Urho3DPlayer Urho3DPlayer.cpp `pkg-config --cflags --libs Urho3D`
 \endcode
 
+\section Scaffolding Project scaffolding
+
+On Unix-like build/host system (Linux, Mac, Raspberry-Pi) with [Rake](https://rubygems.org/gems/rake) ruby gem installed, you can quickly setup all the above by simply using the 'scaffolding' rake task.
+\verbatim
+Usage: rake scaffolding dir=/path/to/new/project/root [project=Scaffolding] [target=Main]
+\endverbatim
+
+As its name implies, this task just creates a basic project structure for your new project. You may also pass optional "project" and "target" parameters besides the "dir" parameter. Otherwise, you will get the default project name (Scaffolding) and target name (Main) in the main CMakeLists.txt. This task copies the Urho3DPlayer.cpp and Urho3DPlayer.h as placeholders for the source files. Normally, you should replace these two files with your own project source files before invoking one of the CMake *.bat or *.sh script.
+
+Unfortunately, all our rake tasks are currently not supported on Windows build/host system yet.
+
 \page Structure Overall structure
 
 The Urho3D engine compiles into one library. Conceptually it consists of several "sublibraries" that represent different subsystems or functionality. Each of these resides in a subdirectory

Source/CMake/Modules/Urho3D-CMake-common.cmake

@@ -476,8 +476,22 @@ macro (setup_target)
     endif ()
 endmacro ()
 
+# Macro for checking the SOURCE_FILES variable is properly initialized
+macro (check_source_files)
+    if (NOT SOURCE_FILES)
+        message (FATAL_ERROR "Could not configure and generate the project file because no source files have been defined yet. "
+            "You can define the source files explicitly by setting the SOURCE_FILES variable in your CMakeLists.txt; or "
+            "by calling the define_source_files() macro which would by default glob all the C++ source files found in the same scope of "
+            "CMakeLists.txt where the macro is being called and the macro would set the SOURCE_FILES variable automatically. "
+            "If your source files are not located in the same directory as the CMakeLists.txt or your source files are "
+            "more than just C++ language then you probably have to pass in extra arguments when calling the macro in order to make it works. "
+            "See the define_source_files() macro definition in the Source/CMake/Modules/Urho3D-CMake-common.cmake for more detail.")
+    endif ()
+endmacro ()
+
 # Macro for setting up a library target
 macro (setup_library)
+    check_source_files ()
     add_library (${TARGET_NAME} ${ARGN} ${SOURCE_FILES})
     setup_target ()
 
@@ -502,6 +516,7 @@ macro (setup_executable)
     # Parse extra arguments
     cmake_parse_arguments (ARG "NODEPS" "" "" ${ARGN})
 
+    check_source_files ()
     add_executable (${TARGET_NAME} ${ARG_UNPARSED_ARGUMENTS} ${SOURCE_FILES})
     if (ARG_NODEPS)
         define_dependency_libs (Urho3D-nodeps)