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.