When the command is invoked without an explicit build directory path we
compute it from the source directory path.  When either the source or
build tree is the root of a Windows drive letter it will have a trailing
slash.  Handle slashes consistently when substituting the current output
directory for the current source directory.

While at it, use cmSystemTools::IsSubDirectory instead of FindLastString
to verify that the source directory is a subdirectory.

Inspired-by: Graham Menhennitt <graham@menhennitt.com.au>