Define include, share, and doc environment variables

[lionel-]

Addresses posit-dev/positron#3637.

Positron Notes

Release Notes

New Features

• N/A

Bug Fixes

• The following environment variables are now set in the same way that R does:

  • R_SHARE_DIR
  • R_INCLUDE_DIR
  • R_DOC_DIR

  This solves a number of problems in situations that depend on these variables being defined (Ark: Set R_INCLUDE_DIR and R_SHARE_DIR envvars? positron#3637).

QA Notes

This should not fail:

stopifnot(
  nzchar(Sys.getenv('R_SHARE_DIR')),
  nzchar(Sys.getenv('R_INCLUDE_DIR')),
  nzchar(Sys.getenv('R_DOC_DIR'))
)

We test for these on our side.

[lionel-]

So that r_command() works properly when the currently active R is not the same as the one on the path.

[DavisVaughan]

Split into r_command() and r_command_from_path() or r_home_from_path() depending on what makes sense