configure: minor: Rephrase and reformat some messages

Make sure all messages use a max. text width of 70 characters (in order
for them to be easily pasteable into an email), remove a trailing
whitespace and reword some bits.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
build: minor: Mention proper versioned tag in release announcement

The tags created by the 'release' target have a 'v' prefix, so add it to
the verification command mentioned in the generated release message.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>