The aim of this ticket is to allow Sage to use the system's R, instead of building its own.

It should work for:

• sage -R command
• Sage's r interface
• Sage's rpy2 Python package
• Sage's jupyter IRkernel

Similarly to the way one can decide to use the system ATLAS when building Sage,
on the user side, building Sage so that it uses the system R is achieved by setting
the SAGE_R_LIB environment variable.

To test this ticket, you should at least:

• install R on your system (e.g. sudo apt install r-base)
• reinstall r and rpy2 with SAGE_R_LIB set:

export SAGE_R_LIB=/usr/lib/R/
sage -f r
sage -f rpy2
make build

• test Sage R command (this should/might show a different version than 3.4.4):

sage -R

• test Sage's R interface:

sage -t --long src/sage/interfaces/r.py

• test rpy2:

sage -t --long src/sage/stats/r.py

CC: @EmmanuelCharpentier @embray @kiwifb @kcrisman @slel @timokau @orlitzky

Component: packages: standard

Keywords: R, sdl

Author: Thierry Monteil

Branch/Commit: u/tmonteil/allow_the_use_of_system_s_r_installation @ eed2ea3

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/25503