Update doc target in Makefile to check roxygen2 version

* Makefile (doc): Raise an error if roxygen2 version not equals the
  version used by git2r. Currently 4.0.1
Refactoring checkout methods

* R/checkout.r: Refactoring of checkout methods to enable extraction
  of the name of the git2r C function in .Call. The purpose is to
  enable a script to extract the names and check that they are
  prefixed with 'git2r_'.
Add check for 'git2r_' prefix in build Makevars script

* tools/build_Makevars.r: Raise an error if missing 'git2r_' prefix in
  git2r C functions.