Fix the way of getting Rinternals header path

`file.path(R.home(), "include", "Rinternals.h")` may lead to wrong paths such as `/usr/lib/R/include/Rinternals.h` on some distributions like Ubuntu. 

`file.path(R.home("include"), "Rinternals.h") `

seems to be a better way, which also takes `R_INCLUDE_DIR` env. variable into account.