Makefile: add/remove comments at top and tweak whitespace

Makefile: add/remove comments at top and tweak whitespace

The top of the Makfile is mostly separated into logical steps like set
default configuration, set programs etc., but there's some deviation
from that.

Let's add mostly comments where they're missing, remove those that
don't add anything. The whitespace tweaking makes subsequent patches
smaller.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>