XMake / XRepo support

[Arthapz]

Hi,
Just want to say that i added the library to XRepo (the XMake Dependency repos) :D

xmake-io/xmake-repo#1631

xrepo install llfio

(to make it work, i ported the build system to XMake, as the CMake was a bit complicated)

[ned14]

Thanks for the contribution.

Did I see some patching of the sources in there? A PR so they could go into trunk would be nice.