Turns out, I had the Makefile syntax wrong.

The error message I was seeing came from a regular expression in dpkg itself excerpted here:

         unless (/^([a-z][a-z0-9_-]*)(?:=(S*))?$/) {

             warning(g_('invalid flag in %s: %s'), $source, $_);

             next;

         }

I re-read that regular expression a million times before I realized that the second %s in the warning does not have quotes around it, which means in the error message:

dh: warning: invalid flag in DEB_BUILD_OPTIONS: "parallel=4"

The quotes around "parallel=4" are coming from the environment variable itself not from the format string.

The fix, is to change

export DEB_BUILD_OPTIONS="parallel=4"

to

export DEB_BUILD_OPTIONS = parallel=4

because when setting the environment variable, make is preserving the quotes in the first version. When make is doing the string interpolation, it makes no practical difference because the quotes get eaten by the shell, but when a subprocess reads the value from an environment, the " characters are preserved.

#!/usr/bin/make -f

export WITH_QUOTES="parallel=3"

export WITHOUT_QUOTES = parallel=3



all:

    bash -c 'printf "WITH_QUOTES=%q

" "$$WITH_QUOTES"'

    bash -c 'printf "WITHOUT_QUOTES=%q

" "$$WITHOUT_QUOTES"'

bash -c 'printf "WITH_QUOTES=%q

" "$WITH_QUOTES"'

WITH_QUOTES="parallel=3"

bash -c 'printf "WITHOUT_QUOTES=%q

" "$WITHOUT_QUOTES"'

WITHOUT_QUOTES=parallel=3

Having the actual value of the environment variable be "parallel=4" not parallel=4 both triggered the warning, and broke this regular expression that dh uses to parse it (note leading ^):

        if ($opt =~ /^parallel=(-?d+)$/ && $wanted eq 'parallel') {

            return $1;

        }