python/CMakeLists.txt: Fix install destination

[podsvirov]

Added CMAKE_INSTALL_PYLIBDIR cached variable.
Default value retrivet from Python3_EXECUTABLE.

Current install destination Python3_SITELIB (absolute path from FindPython3 module).
An absolute path is not acceptable when creating an installation package.

[jmr]

Calling this CMAKE_ seems a bit strange. I would assume that's only for variables defined by CMake itself.

Can you point me at some CMake Python documentation or a canonical example? It seems no one has used this variable name before.

https://www.google.com/search?q=%22CMAKE_INSTALL_SITELIBDIR%22
https://github.com/search?q=CMAKE_INSTALL_SITELIBDIR

[jmr]

Also, what problem are you trying to solve? Use of this variable should also be documented somewhere.

[podsvirov]

I'm trying to do something like there: https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html.

[podsvirov]

@jmr, please review #186 (this should clarify my motivation).

[jmr]

It seems like there should be a relevant directory somewhere here:
https://cmake.org/cmake/help/latest/module/FindPython3.html

How do other swig/python/windows/cmake projects do it?

[jmr]

Does #186 make this unnecessary?

[podsvirov]

These changes are not directly related to the #186. These changes solve the problem of hard-coded absolute installation path for the python module. Absolute install paths add additional problems when creating packages for distribution (in this case, it becomes necessary to install to a path other than the default).

[jmr]

This has merge conflicts. Is it still relevant?

[jmr]

What do other Python people think? @andyneff

[andyneff]

It seems fine to me. Normally I would try to do the string manipulation in cmake, but I don't see an obvious way to have the python's sys.prefix without calling python (which will often be different from the normal prefix /usr).