replace the python version finding mess with cmake builtins

[nilsnolde]

fixes #2929

it uses the "new" CMake builtin approach to get the right python lib path from the actual python installation used to build the bindings. Meaning it'll install now to /usr/lib/python3/dist-packages/valhalla instead of /usr/local..., but that's actually better IMO

[kevinkreiser]

Suggested change

# TODO: fix https://github.com/valhalla/valhalla/issues/2929 and update this line

[kevinkreiser]

i kind of prefer the /usr/local prefix as that is the standard location to install when you build something from source. ie i think its the canonical location for from source builds. but at the same time i like the extreme simplification in the cmakefile. maybe we can merge this can get the /usr/local back later

[nilsnolde]

Well, yeah I agree for valhalla (and other self-contained/-built apps), but if you think about it more, it actually makes sense to let the python framework you're using to build the bindings tell you where to install them. It installs to /usr/local if your python installation is also at /usr/local. And if you force the install to /usr/local, a regular python won't be able to find it by default, you'd have to alter the sys.path. An analogy would be if someone wanted to build a valhalla add-on, but strictly installs it to /usr instead of letting the already (wherever) installed valhalla decide where to put the plugin so it can even find it. Plus the really nice simplification:)

[kevinkreiser]

i still think /usr/local is quite standard:

Python 3.6.9 (default, Jan 26 2021, 15:33:00) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
Could not open PYTHONSTARTUP
FileNotFoundError: [Errno 2] No such file or directory: '/home/kkreiser/.pythonrc'
>>> import sys
>>> print(sys.path)
['', '/usr/lib/python36.zip', '/usr/lib/python3.6', '/usr/lib/python3.6/lib-dynload', '/usr/local/lib/python3.6/dist-packages', '/usr/lib/python3/dist-packages']

but as i mentioned im totally cool with tackling that later

[nilsnolde]

Huh, you're right, thought I checked it on ubuntu. Maybe I did it on my arch, where /usr/local is not in the sys.path (or in the default linker paths, guess they go together).. I have less strong feelings about it in that case:)