PyPI: Only use major version for libnvme.so

[martin-belanger]

When building a pip (PyPI) package, do not set the libnvme.so version to major.minor.patch (e.g. libnvme.so.3.0.0), but instead just set it to the major (e.g. libnvme.so.3). pip/wheel do not care about the full versioning, and since symbolic links cannot be installed via pip (i.e. libnvme.so.3 -> libnvme.so.3.0.0), it's better to just skip the major.minor.patch versioning for PyPI.

To support this we're introducing a new config flag "pypi" to let meson know we're building for pip/PyPI.

@igaw - After doing more testing I found that the previous fix to set the symbolic links did not work. This PR reverts the previous change and provides a simpler fix more in line with PyPI packages.

[igaw]

Just as idea: doesn't PyPi set a build environment variable? Couldn't we use that one as input? Or is this a stupid idea?
Also would it make sense to toggle the other options (-Dnvme=disabled...) based on this one?

[martin-belanger]

I'm not familiar with the PyPI build environment variables. Heck, I did not know anything about building PyPI until a few days ago... 😄. But I'm curious to know what you have in mind.

Yes, it does make sense to toggle all the options based on the -Dpypi being set. I can work on that.

[martin-belanger]

@igaw - Note that toggling the other options based on -Dpypi=true is not 100% foolproof. That's because we can't actually overwrite the value of an option returned by get_option().

For example, we can overwrite options like this:

want_pypi = get_option('pypi')
if want_pypi
   want_tests = false
   want_nvme = false
   want_libnvme = true
   etc...
else
   want_tests = get_option('tests')
   want_nvme = get_option('nvme').disabled() == false
   want_libnvme = get_option('libnvme').disabled() == false
   etc...
endif

We just need to make sure to use if want_<option> instead of if get_option('<option>') in all the meson.build files.

Would you like me to proceed with this change?

[igaw]

Okay, sounds like asking for trouble. Let's go with your solution. If it works we can still clean it up if needed.