Plugin uses GNU-specific options with 'cp', 'rm' and 'rmdir'

This means that it fails on macOS unless you have GNU cp/rm/rmdir ahead of the BSD ones in $PATH. For example:

```
cp: illegal option -- -
usage: cp [-R [-H | -L | -P]] [-fi | -n] [-aclpsvXx] source_file target_file
       cp [-R [-H | -L | -P]] [-fi | -n] [-aclpsvXx] source_file ... target_directory
make: *** [foo/deps/elvis_mk/plugins.mk:39: foo/elvis] Error 64
```