PR: Improve calls to libpng-config on Ubuntu/Debian

[andfoy]

Fixes #2392

This PR detects if libpng was installed via apt on Ubuntu/Debian Linux distributions, if so, then the call libpng-config --libdir is disabled, since that flag is not available on those distributions.

[codecov]

Codecov Report

Merging #2398 into master will not change coverage.
The diff coverage is n/a.

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##           master    #2398   +/-   ##
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  Coverage   70.65%   70.65%           
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  Files          94       94           
  Lines        7897     7897           
  Branches     1241     1241           
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  Hits         5580     5580           
  Misses       1934     1934           
  Partials      383      383           

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[fmassa]

Thanks!

[t-vi]

This breaks building on Debian testing and unstable. These do not feature a VERSION field in /etc/os-release.
And what is the version used for?

[fmassa]

@t-vi the version is necessary because we need a recent-enough version of libpng, otherwise we have errors such as #2301 (comment), which requires libpng 1.6 or higher. I misread the VERSION you were talking about. I don't know, @andfoy can you comment on Thomas question?

@andfoy can you have a look at @t-vi comment?

[t-vi]

Sorry, I was to terse there! (After waiting for compiling PyTorch to finish I was too impatient.)
So if you don't need the VERSION from os-release, I'd just rip it out the code reading it, but would not want to do that without asking. If you (plan to) use it, we could make up a default.

[fmassa]

I think we needed that to fix a behavior in another PyTorch CI, but @andfoy can comment on it. I'm all in for simplifying as much as possible this pipeline.

[andfoy]

Hi @t-vi, thanks for reporting this problem, I think I can simplify the detection logic, so that we don't depend on reading distribution files that might change between versions