include: make header guards more sane The C++ standard[1] says: Each identifier that contains a double underscore __ or begins with an underscore followed by an uppercase letter is reserved to the implementation for any use. [1]: https://timsong-cpp.github.io/cppwp/n4659/lex.name#3.1

<?xml version="1.0"?>
<mesh name="mesh_root">
	<!-- here is a mesh node -->
	some text
	<![CDATA[someothertext]]>
	some more text
	<node attr1="value1" attr2="value2" />
	<node attr1="value2">
		<innernode/>
	</node>
</mesh>
<?include somedata?>