refactor with LuaExpat

[fperrad]

dbus and dbus-broker are linked with expat (XML parsing C library).

LuaExpat is available since long time with the Debian package lua-expat.

So, xml_fromfile and xml_fromstr could be refactored in plain Lua.

By this way, the native part of lsdbus is focused on sd-bus binding.

[kmarkus]

Thank you. The issue with this is that we're use this module on small embedded systems that has libmxml (and libexpat) available but not the lua-expat bindings. So I'd much prefer to keep the built-in serialization support to avoid another dependency. I wouldn't mind switching to libexpat instead of libmxml however.

[fperrad]

I see your point.
I also target embedded Linux.
I use Buildroot which comes with a LuaExpat package (see https://git.busybox.net/buildroot/tree/package/luaexpat/luaexpat.mk).

[kmarkus]

One possibility to support both options would be to add a CMAKE option DISABLE_MXML. If enabled, mxml is not required and checked for and the respective functions are ifdef'ed out. In init.lua, we can test if xml_fromstr and xml_fromfile are defined and if not we try to load and assign the LuaExpat based implementation (preferrably from it's own module).

[fperrad]

The idea to support both via conditional compilation sounds good.

But I prefer a option named WITH_MXML or ENABLE_MXML, this avoids double negation (and headache) in the logic.

Go ahead please, because I really master not CMAKE.