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I never noticed this because unless I am debugging a build problem I use "make -s", which hides all the commands lines, and lets you see compiler warnings etc much easier.
Very briefly testing with Gnu Make and BSD Make, your patch doesn’t seem to do anything too disastrous. BTW BSD Make seems to treat the existing python-config target comments as Make comments without any output, while Gnu Make treats them as shell comments and outputs them.
However it seems that other Make implementations might take your comments in the middle of a list of commands as a sign to end the list of commands. See <http://pubs.opengroup.org/onlinepubs/9699919799/utilities/make.html>:
"An empty or blank line, or a line beginning with '#', may begin a new entry."
Maybe it would be best to move these comments to before the target rules, with clarification, e.g. "Substitution happens [in the first sed command] . . . [The second sed command] replaces . . ."