allow the prefix (./usr) to be a symlink

[treeswift]

• make sure that absolute paths written into the prefix are realpath-sanitized
• make sure that re-targeting the ./usr symlink does not trigger a rebuild (i.e. make sure make considers target timestamps rather than symlink timestamps)
• make sure that the build system and the build target are otherwise properly separated (i.e. no state of ./usr is tracked or implied outside of it)

Low priority, but nice to have.