Building R

[hturner]

The guide assumes that there is no previous attempt and the users starts from a clean state. But it might be nice to mention something about how to make sure it is a clean build.
_original comment by @llrs on #124 _

On Linux this can be done by deleting (all files under) the build directory - this is an advantage of using a separate build directory.

On Windows, the build is done within the svn directory - best practice to delete the local copy and check out from scratch?

[llrs]

Reviewing this, it if done in the svn directory there is this command svan cleanup to avoid delting the local copy and start from scratch. It deletes anything in the directory that is not tracked by svn and reverts those that are tracked.