[Needs review] Fix python site-packages dir

[NTULINUX]

Fixes the need for PYTHONPATH on Gentoo

[NTULINUX]

Looks like this needs to be fixed to account for Debian..

dh_install: error: missing files, aborting

Ah, looks like various parts of the tree are also hard-coded to use /usr/lib/python3/dist-packages

docs/src/plasma/qtplasmac.adoc:/usr/lib/python3/dist-packages/qtvcp/designer/install_script
lib/python/qtvcp/designer/install_script:    if [ -f /usr/lib/python3/dist-packages/qtvcp/plugins/qtvcp_plugin.py ]; then
lib/python/qtvcp/designer/install_script:        pifile=/usr/lib/python3/dist-packages/qtvcp/plugins/qtvcp_plugin.py
lib/python/qtvcp/designer/install_script:        echo -e '\n'"$pifile "'not found in /usr/lib/python3/dist-packages/qtvcp/plugins/'
scripts/setup_designer.in:    if [ -f /usr/lib/python3/dist-packages/qtvcp/plugins/qtvcp_plugin.py ]; then
scripts/setup_designer.in:        pifile=/usr/lib/python3/dist-packages/qtvcp/plugins/qtvcp_plugin.py
scripts/setup_designer.in:        echo -e '\n'"$pifile"' not found in /usr/lib/python3/dist-packages/qtvcp/plugins/'

Did not see these. Any tips? The issue is that depending on the exact version of python that debian uses will affect the installation path, which will vary from each debian release, that's the tricky part to make this work across all versions. Not sure on a clean way to avoid the hard-coded variables either, as those will also change.

[BsAtHome]

Is there a distro independent way to find out where python packages need to be installed?

[NTULINUX]

Well that's what this should do:

https://github.com/LinuxCNC/linuxcnc/blob/master/src/m4/ax_python_devel.m4#L340

That's where I got PYTHON_SITE_PKG from. There's also python -c "import site; print(site.getsitepackages())"

That's going to change though distro to distro... The commands themselves are universal, the result is not.

PYTHON_SITE_PKG works but debian/linuxcnc.install.in needs to account for it. It is hard-coded to usr/lib/python3 as well as other parts of the tree.

git grep -e "usr/lib/python3"

Using PYTHON_SITE_PKG works for both a system-wide install though, and run-in-place.

I just checked in my Trixie VM and PYTHON_SITE_PKG is working as intended (both rip and system-wide install.)

Another problem is accounting for Fedora. PYTHON_SITE_PKG works there too but the hard-codes need to be fixed with a simple one-linger, not some 10 line script from autotools. Luckily, this works:

python3 -c "import site; print([p for p in site.getsitepackages() if p.startswith('/usr/lib') and '/local/' not in p and '/lib64/' not in p][0])"

Now I need to fix the other hard-codes but the upcoming code might be wrong..

[NTULINUX]

This might be ready now. Not 100% certain.