Unable to find header files

I've worked through the README and when it comes time to run `$ make` I receive the error `fatal error: 'fluent-bit/flb_info.h' file not found`. I am setting `-DFLB_SOURCE` when calling `cmake` and it finishes without error. I feel like there's another piece of missing information to get things up and running.

Start with the README instructions and the CMake invocation using -DFLB_SOURCE, then reproduce the `$ make` failure. Trace why `fluent-bit/flb_info.h` is not available during the build; done means the documented setup completes without the missing-header error.

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