Detect if R is installed and if so, pass it to the build

[NickSeagull]

With the inline-r package, one can write R code using oxford brackets like so:

let x = 2
let y = 3
myArray <- [r| c(1, x_hs, y_hs) |]
[r| plot(cars) |]

The only requirement is to pass the R binaries and include directories to stack build by using the stack command line options --extra-lib-dirs and --extra-include-dirs.

We could check if the user has the R_LIB_DIR and R_INCLUDE_DIR set and if R is in the PATH. If this is true, another build command would be used.

[NickSeagull]

Not necessary as stack checks standard lib dirs and includes, also in case that the user wants it, they just can add it to stack.yaml. Closing