Detecting R Version in Rcpp/arrayfire in makevars

I am new to building R packages so I need some help :) I am using Rcpp/arrayfire and want a line in my makevars file to detect the users R version. Currently I have it set in 4.0, but I anticipate users having different R versions.

If this question has been answered, I apologize for not finding one!

Here are my global variables in the makevars file

R_VERSION = 4.0
AF_CXXFLAGS = -I/opt/arrayfire/include
AF_LIBS   = -L/opt/arrayfire/lib -laf -Wl,-rpath,/opt/arrayfire/lib /Library/Frameworks/R.framework/Versions/$(R_VERSION)/Resources/library/RcppArrayFire/libs/RcppArrayFire.so -Wl,-rpath,/Library/Frameworks/R.framework/Versions/$(R_VERSION)/Resources/library/RcppArrayFire/libs

The usual workflow is to use a script called configure (which can be written in any language) which 'detects this' and then writes or alters src/Makevars accordingly.

If you know a little about make or want to learn it you can also do in a Makefile -- and our script src/Makevars is one. So something like this saved in a file Makefile

RVER = `Rscript -e 'cat(R.Version()$$major)'`

SOMEDIR = "/opt/foo/bar/"${RVER}"/some/more"

all:
    @echo Using ${SOMEDIR}

results in

$ make 
Using /opt/foo/bar/4/some/more
$ 

Edit And if you wanted just "4.2" out of the version, one way might be

> gsub("(\\.\\d)?$", "", format(getRversion()))
[1] "4.2"
> 

Edit 2 As a full Makefile it becomes

#RVER = `Rscript -e 'cat(R.Version()$$major)'`
RVER = `Rscript -e 'cat(gsub("(\\\\.\\\\d)?$$", "", format(getRversion())))'`

SOMEDIR = "/opt/foo/bar/"${RVER}"/some/more"

all:
    @echo Using ${SOMEDIR}