Please make python link conditional on an cmake variable

[yurivict]

It should be possible to turn off python bindings.

Further, python bindings should be buildable separately from the C++ library, because system can have several python versions installed at the same time, and each one can have s2geometry bindings built for it.

[spaetz]

I wanted to package s2geometry as a library for Debian, and requiring python include files even when not needing the python interface was a killer. Some inspiration to make python optional can be found here: https://github.com/rinigus/s2geometry/blob/main/CMakeLists.txt

Sorry, I don't know enough CMake magic to really provide a nice patch.

[spaetz]

Sorry, I see there is even a MR to provide this at #170

[jmr]

Fixed by #189