install geos.h header file in cmake builds

[tbussmann]

since the change from autoconf to cmake this header was not longer installed with make install

[pramsey]

On the one hand I like the idea of geos.h pulling in the "official C++ API", we've deprecated the C++ since 15 years ago.

https://github.com/libgeos/geos/blame/main/include/geos/geom.h#L144

Wonder what @dbaston thinks?

[dbaston]

Sounds fine to me, once we decide what the "C++ API" is :)

[pramsey]

Yeah! I was wondering if these headers even still worked. Like, can I write the C++ example apps up in terms of just "include geos.h and go". I have no idea how deep the dependency graph reaches once you include it.

[tbussmann]

I don't know if and how often that header is used for quick access to the deprecated C++ API, but no longer providing it seems to be an unintended change introduced by the switch to the cmake-only build system. If you conclude leaving this out helps to transition away of it, the readme.md should be adopted, at least.

[pramsey]

Taking this one in, I think it's good for the future (maybe) but good to retain continuity with the past until/if we deliberately change.