* Makefile improvements

- Remove invocations of `pwd` where not necessary
- Add invocations of `pwd` where necessary
- Replace PREFIX?= with PREFIX=

(The removal of ? in PREFIX?= makes this setting not susceptible to
unintentionally taking the value from the environment variable.
The correct way to set PREFIX is by doing 'make install
PREFIX=/path'
(as opposed to 'PREFIX=/path make install').)

- Do not run link (which requires admin privileges) in default
  target (build)
- On install, remove destination file before copying the binary
  (solves problem when a symlink already exists and copying fails)
- Make build depend on lib/
- Run shards only once, if lib/ doesn't exist
- Better qualify $OUT_DIR
- Add missing echo

* Make link targets depend on build, just like install