Makefile: Don't use `echo`'s `-e` option for portability.

Many systems (including Debian, Ubuntu and derivatives like Linux Mint) use
Dash as a noninteractive version of `/bin/sh`, invoked by `make`.

Dash's `echo` command doesn't understand the `-e` option and this generates
spurious output when running `make`.  See [a bugreport][0] for one of the
many instances of this bug/feature in action.

[0]: https://bugs.launchpad.net/ubuntu/+source/dash/+bug/72167