How to get use CalendarAstronomer from ICU

I would like to use the CalendarAstronomer class from ICU to calculate the sunset/sunrise values for a given location.

The API is good and clean, but the necessary file astro.h is not installed. Neither using apt-get nor building ICU by myself. What is wrong here? Are their any special components I could not find out about that are needed for the CalendarAstronomer class?

BTW: The main reason is the pretty liberal license of IC开发者_如何学运维U. I found several code samples calculating sunset/sunrise values, but the licenses are often now clear. So here is an alternative question: Are their other libraries/code samples using a liberal license (Apache, BSD) calculating the sunset/sunrise in C++?

As you said, astro.h is internal. That's the right answer. But, it's open source as you noted…