makefoo.m4: download makefoo from github if not found
makefoo.m4: notice when --with-makefoo-dir was used
merge of '4003d6323fff5b6d6f36413c6e7dfc1645ad9272'
     and 'e1eb4b995aa26829097ed5ef6909e2342ffcaa9c'
* debug.mk: Makefile debug module added (show-defs and show-outputs targets) - included by default
* rest: expose key variables