Suggest moving R configuration querying into a separate crate

[bnaras]

Just recently had a good reason to use some code from here for a package for R BLAS+LAPACK . Thank you.

It appears to me some of the code in the build.rs here really belongs in another crate that would be very useful in build scripts. I've put up the implementation that I needed for R CMD config here.

Combining it with the RHOME and InstallationPaths facilities available here, plus adding a simple Rscript for ability would increase its usefulness I think.

If there is interest, I am happy to transfer this over to extendr and continue development. Please let me know.

[CGMossa]

There are several limitations in the current Rust ecosystem to "inherit" configurations from one build.rs to another. Furthermore I've spent extensive effort attempting to bind the R-API headers onto a more idiomatic rust representation, and it has not been successful. We won't be doing this at this time unfortunately.