[R] Alternate bindings for when libarrow is not found

[asfimport]

This will also allow the package to build and install successfully on hosts where the arrow C++ library is not present, which will enable us, among other things, to provide an install_arrow() function similar to other R packages that have big external dependencies.  

Reporter: Neal Richardson / @nealrichardson
Assignee: Romain Francois / @romainfrancois

_{Note: This issue was originally created as ARROW-5499. Please see the migration documentation for further details.}