[R-package] remove unused variables in configure.ac

[jameslamb]

Short summary

This removes some unused variables in R-package/configure.ac.

Longer Summary

The CRAN build of the R package contains a file configure, which is a shell script that run whenever the package is installed. It looks around on the system to figure out things like "is OpenMP installed", then fills out variables in a templated Makefile (R-package/src/Makevars.[w]in), which is then used to compile the library.

configure is created from a file R-package/configure.ac using autoconf.

When configure.ac was originally added, I copied and pasted some code from other examples and "Writing R Extensions". That included this snippet:

CC=`"${R_HOME}/bin/R" CMD config CC`
CXX=`"${R_HOME}/bin/R" CMD config CXX11`
CFLAGS=`"${R_HOME}/bin/R" CMD config CFLAGS`
CPPFLAGS=`"${R_HOME}/bin/R" CMD config CPPFLAGS`

Now that I understand this better, I can see that CXX, CFLAGS, and CPPFLAGS are unused...they're not referenced in the AC_SUBST() calls used to replace template string in src/Makevars.[w]in.

How I tested this

I ran the following to test the package, before and after this change:

sh build-cran-package.sh
R CMD INSTALL lightgbm*.tar.gz

The compiler and linker flags used in both cases were identical, and the library installed successfully, so I'm confident that these variables can be removed.

[StrikerRUS]

Very weird that CC is used while CXX is not... But if you are confident, OK!

[jameslamb]

Very weird that CC is used while CXX is not... But if you are confident, OK!

ah!!! I think it should be CXX!

We do the malloc check with a C++, not C, compiler, in CMake builds:

https://github.com/microsoft/LightGBM/blob/master/CMakeLists.txt#L241

But that change will require a bit more testing, so I'll do it as a separate PR.

[jameslamb]

Very weird that CC is used while CXX is not... But if you are confident, OK!

re-reading this, I realize I need to clarify....configure.ac is only used to template Makevars.in so that packages can override R's defaults. So R will use the compiler identifier by R CMD config CXX when compiling the library...we just didn't need to access it in configure.ac.

BUT...since we do a special check for malloc.h, we need a compiler for that test program...and that should be using CXX. I'll change that in another PR.

[github-actions]

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