/usr/lib/x86_64-linux-gnu/glibmm-2.4/proc/m4/initialize_gio.m4 is in libglibmm-2.4-dev 2.39.93-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | dnl
dnl Initializations for giomm C++ types from Gio C types.
dnl
dnl UnixFDList
_INITIALIZATION(`Glib::RefPtr<UnixFDList>&',`GUnixFDList*', `$3 = Glib::wrap($4)')
|