/usr/lib/x86_64-linux-gnu/glibmm-2.4/proc/m4/initialize.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 | dnl Other libraries, such as libgnomeuimm, can provide their own initialize.m4
dnl files, maybe choosing to include the same files as this one.
include(initialize_glibmm.m4)
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