gnu: r: Add R_LIBS_SITE to native-search-paths.

* gnu/packages/statistics.scm (r)[native-search-paths]: Add
  specification for "R_LIBS_SITE" variable.

gnu/packages/statistics.scm

@@ -105,6 +105,10 @@ (define-public r
        ("pcre" ,pcre)
        ("readline" ,readline)
        ("zlib" ,zlib)))
+    (native-search-paths
+     (list (search-path-specification
+            (variable "R_LIBS_SITE")
+            (files (list "site-library/")))))
     (home-page "http://www.r-project.org/")
     (synopsis "Environment for statistical computing and graphics")
     (description