/usr/lib/x86_64-linux-gnu/glibmm-2.4/proc/m4/convert.m4 is in libglibmm-2.4-dev 2.42.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# Other libraries, such as libgnomeuimm, can provide their own convert.m4 files,
# Maybe choosing to include the same files as this one.
include(convert_glibmm.m4)
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