Remove a useless printing.

	modified:   test/CMakeLists.txt
Update the 'print_variable' macro source file.

	modified:   config/cmake/print_variable.cmake
Add a new CMake macro.

	new file:   config/cmake/add_doxygen_input.cmake
Update the documentation building CMake script.

	modified:   CMakeLists.txt
	modified:   doc/CMakeLists.txt
	modified:   doc/doxygen.config.in
	modified:   src/c/CMakeLists.txt