CMake git submodule macro

git.cmake

function(add_submodule path url)
	find_program(GIT git CACHE)
	message(STATUS "git submodule ${path}")
	execute_process(ERROR_QUIET WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
		COMMAND ${GIT} submodule add ${url} ${path}
		COMMAND ${GIT} submodule update --init --recursive ${path})

	if (NOT ARGV2 STREQUAL "") # a tag or a branch
		get_filename_component(submodule_directory "${path}" ABSOLUTE)
		execute_process(COMMAND ${GIT} checkout ${ARGV2} OUTPUT_QUIET WORKING_DIRECTORY ${submodule_directory} ERROR_QUIET)
	endif()

	get_filename_component(dirname ${path} DIRECTORY)
	if (dirname STREQUAL "")
		add_subdirectory(${path}) # submodule is not being nested; it should already have a CMakeLists.txt
	else()
		get_filename_component(submodule_cmake_file "${dirname}/CMakeLists.txt" ABSOLUTE)
		if(NOT EXISTS ${submodule_cmake_file})
			message(STATUS "Creating empty ${dirname}/CMakeLists.txt")
			file(APPEND ${submodule_cmake_file} "cmake_minimum_required(VERSION 2.8.4)\nproject(${dirname})\n")
		endif()
		add_subdirectory(${dirname})
	endif()
endfunction()