Apparently this request (Enable dynamic modules) doesn't really make sense for Debian before 26.2 is packaged; with 26.1 and earlier, it can't be used outside the Emacs source tree. But with 26.2 (which I assume we'll see soon now that Buster is released), "[t]he emacs-module.h file is now installed in the system-wide include directory as part of the Emacs installation", according to the NEWS file in GNU's git.
(You probably know all this; I just thought it should be on record in the BTS report.) cheers
