Install mlpack and cereal headers in R package

[rcurtin]

This handles #3486.

By putting the mlpack and cereal headers into inst/include/, they will be installed as part of the R package installation process. I also had to update Makevars to correctly include the headers (in the same way that's done in the Makevars of RcppEnsmallen).

[eddelbuettel]

Just got in from 25 miles on the bicycle where I realized ... we are doing this likely wrong :) When you install the headers into src/, they should go directly to inst/include/ and src/Makevars should get a -I../inst/include to use them during package build. Anyway... Will review now.

[eddelbuettel]

Excellent stuff -- 🚢

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍

[zoq]

Looks good to me.

[rcurtin]

Yep, it took me a minute to debug the failing test (unrelated to this PR), but now that it's all green I'll go ahead and merge. 👍

[eddelbuettel]

Thanks so much for the persistence! Should we make a 4.1.1 release for CRAN?

[rcurtin]

I was thinking of waiting on #3487 and other nearly-ready-to-go PRs to be merged, and then releasing the next version of mlpack (thus updating CRAN too). So, within the next week or so?

[eddelbuettel]

That works too and is a wee bit cleaner :)

[conradsnicta]

I was thinking of waiting on #3487 and other nearly-ready-to-go PRs to be merged, and then releasing the next version of mlpack (thus updating CRAN too). So, within the next week or so?

@rcurtin Suggest to label the new release as mlpack 4.2, instead of 4.1.1. The changes are more than just bug fixes -- there are new features. On top of that, people like new shiny things with bigger numbers.

[rcurtin]

Agreed, 4.2.0 is just fine given the new features.