dist/prepare: always print '?' for empty values in verbose mode
dist/prepare: add some explanatory comments
dist/prepare: create $OBJDIR if missing, even if $out not set
dist/prepare: warn about fallbacks being used
dist/prepare: add $HOSTNAME to variable list
dist/prepare: add fallbacks for $OSTYPE, $HOSTNAME
dist/prepare: add usage text
Update CACHEDIR.TAG to mention the creator