Replaces some @echo recipes by @printf.

The echo command does not understand the -n argument on Mac OS X.
This is due to the fact that:

  - Makefile calls /bin/sh to execute a command
  - in Mac OS X, /bin/sh is a link to bash
  - in Mac OS X, bash is compiled with --enable-strict-posix-default

Therefore, the echo command does not have the -n argument.

printf on the other hand is a builtin function that works well on Mac OS X and Linux.