Building from commit f3f9f9b, I get the following when running
./configure --sysconfdir=/etc --enable-maintainer-mode
...
/Users/leif/src/gpg/gnupg/build-aux/missing: Unknown `--is-lightweight' option
Try `/Users/leif/src/gpg/gnupg/build-aux/missing --help' for more information
configure: WARNING: 'missing' script is too old or missing
...
The file 'build-aux/missing' is the source of this message.
The attached version of 'build-aux/missing' is taken from the automake source repo at
http://git.savannah.gnu.org/cgit/automake.git/plain/lib/missing
f62bbae306c7e1bc28896aab8fe7bfb700a9a33e
Building with this in place corrects this message.
Given that I am merely copying a file from another FSF body-of-work, I don't know if I
am required to make a disclaimer (the file > 10 lines long, as per
https://www.gnupg.org/documentation/bts.html).
If it is required, please consider this my disclaimer to assign any copyright I possess
over this code to the FSF.