Enforce R-package dependencies in build_r.R before compilation begins (#1672)

* Enforce R-package dependencies in build_r.R before compilation begins

* Greatly simplify the previous commit

build_r.R

@@ -5,6 +5,10 @@
 # Sys.setenv("CXX" = "/usr/local/bin/g++-8")
 # Sys.setenv("CC" = "/usr/local/bin/gcc-8")
 
+library(testthat)
+library(roxygen2)
+library(devtools)
+
 # R returns FALSE (not a non-zero exit code) if a file copy operation
 # breaks. Let's fix that
 .handle_result <- function(res) {