Git submodule + CMake

Hi, incredible work on this project ! I'm interested in using it in a static site generator I'm working on, but my initial attempt using the library via git submodule failed to import the headers. Could you add support for the git submodule workflow ?

Thanks

[adamharrison]

I actually do use it as a submodule in my nginx-xapian project, so it is possible. I'll look into this.