configure can now send one line to the project saying that a build
happened and which back ends were detected, so we have some idea how
many people build Aubit and which drivers are still worth maintaining.
It only asks on a terminal, defaults to no, and never runs in a scripted
or CI build. --enable-usage-report sends without asking,
--disable-usage-report never asks, and DO_NOT_TRACK is honoured. The
prompt shows exactly what would be sent, and nothing is sent when
building or running programs afterwards - only here, and only on a yes.
Note the prompt reads /dev/tty rather than stdin. Autoconf detaches
stdin near the top of every generated configure
test -n "$DJDIR" || exec 7<&0 </dev/null
so that configure can never block on input, which means a prompt reading
stdin silently does nothing at all.
