[R-package] allow R to pick between -fPIC and -fpic for CRAN package

[jameslamb]

I think we can remove setting CPICFLAGS in Makevars.in and Makevars.win.in in the R package. I noticed in the logs from R Hub (#629 (comment)) that my choioce was being ignored anyway...-fpic is used in many of the jobs.

Re-reading "Writing R Extensions", I see that I probably should have been using CXX11PICFLAGS.

In this PR, I propose just removing CPICFLAGS from these Makefiles and using whatever values R chooses based on the environment it is installed in. That should slightly simplify our configuration and reduce the risk of the CRAN package failing checks on the many obscure environments CRAN uses for testing.

[StrikerRUS]

I'm glad to see configuration simplification! But I'm not happy with that we are getting more and more differences in code compilation for manual builds and CRAN ones. This means we will have to ask users how they installed LightGBM to debug reported issues.

[StrikerRUS]

According to the GitHub Actions logs from CRAN jobs, -fpic is set by default for Linux and macOS and isn't set for Windows.

[jameslamb]

I'm glad to see configuration simplification! But I'm not happy with that we are getting more and more differences in code compilation for manual builds and CRAN ones. This means we will have to ask users how they installed LightGBM to debug reported issues.

I think we will already have to do that anyway, and that the maintenance burden is well worth it. I think our current build process involving cmake can be quite intimidating for the typical R user who is used to just running install.packages()

[github-actions]

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