Prepend PATH with path to RTools on Windows

[rok-cesnovar]

We have seen a few issues from users that turned out to be muliple g++ installed on the system. Typically some MinGw install or something along those lines.

What we could do is try to prepend the path to rtools on the front of PATH (like we do for TBB) which would then force the use of the right compiler.

We would check the usual paths to rtools and if found use it. If the user installed it in some non-typical location, then we would not prepend it.

[jgabry]

I don't know much about this topic, but that sounds ok to me if you think it's a good idea

[andrjohns]

Not sure whether is what you're talking about, but for Windows there's an RTOOLS40_HOME environment variable that gives the install directory for Rtools (I believe the equivalent for version 3.5 and lower is RTOOLS_HOME), which could simplify finding the right compiler

[rok-cesnovar]

Good call. RTOOLS40_HOME is definitely something we could rely on. We do have to double-check if this gets set for all RTools 4.0 installs. Hopefully, there is no way of not setting it. If that is the case, then it should be easy for R 4.0.

[rok-cesnovar]

Ok, silly me. As @andrjohns pointed out, prepending PATH is actually what RTools 4.0 suggests upon installing (see https://cran.r-project.org/bin/windows/Rtools/#putting-rtools-on-the-path)

writeLines('PATH="${RTOOLS40_HOME}\\usr\\bin;${PATH}"', con = "~/.Renviron")

So maybe we should just check if the user did that? And if they did not, do it for them or suggest they do it themselves?

[jgabry]

Sounds good to me