Use proper python module destination

[ghostshadow]

Since the maintainer of the AUR package (https://aur.archlinux.org/packages/opencamlib-git) appears to have difficulties keeping up with patching the python module install path (my install was broken again), I thought it should actually be fixed upstream.
I also noted the issue of the Debian package maintainer (#157 ) and this should improve the situation there as well.

The "FindPython3" CMake module already provides variables containing the paths for installing modules into (https://cmake.org/cmake/help/latest/module/FindPython3.html, Python3_SITELIB and Python3_SITEARCH), so why not use these directly? This way it should be a lot more portable and not require the execute_process.

Note: Since site.getsitepackages() only contains one element on my system, the index [-2] is out of bounds resulting in an exception. This caused the python module being installed to /opencamlib, which, I think, is even worse than /usr/lib/opencamlib, as it was in the version before 🥲

[aewallin]

any idea why the CI builds/tests all fail? something changed on github side?

[vespakoen]

I am fine with using Python3_SITEARCH.

Looks like boost download link changed causing the builds to break, I will update it to a newer one and see what happens.