cmeel-m4: Add version 1.4.20 - #4847

GNU M4 wraps its own upstream autotools build (./configure && make &&
make install via ExternalProject_Add) through the cmeel build-backend,
same shape already proven here by cmeel-fmt/cmeel-gmp/cmeel-gklib/
cmeel-cppad/cmeel-coinutils/cmeel-lapack: one has-sitelib=false py3-none
wheel holding only the bin/m4 binary, no Python extension to build per
interpreter. m4 is a build-time tool declared as an extra == "build"
dependency by several other cmeel-* wrappers (cmeel-autoconf/
cmeel-automake/cmeel-gettext/cmeel-texinfo), not a library other
packages link against at runtime.

GPL-3.0-only (GNU m4 itself). Re-read patching-and-licensing.md
directly rather than from memory: the documented GPL procedure (the
collect-gpl-sources job) is scoped to GPL components pulled in from our
own build environment (e.g. the manylinux image's gcc), not to porting
a project whose own upstream source happens to be GPL-licensed. The
actual applicable check is the doc's ordinary two bullets ("wheel
carries upstream LICENSE" + "linked libraries' own licence reqs met"),
same as every other license in this repo. Confirmed against the real
published x86_64 wheel (cmeel_m4-1.4.20-0-py3-none-manylinux_2_28_x86_64.whl,
unzip -l) that it ships no LICENSE/COPYING file at all - a genuine gap
in the upstream wrapper itself, present on every platform, not
riscv64-specific. GNU m4's own release tarball ships COPYING at its top
level (confirmed: tar -tJf m4-1.4.20.tar.xz). This wrapper's
ExternalProject_Add(m4 ...) uses BUILD_IN_SOURCE ON with no explicit
SOURCE_DIR, so CMake's own documented default applies (confirmed
against the real ExternalProject.cmake module source and its
extractfile.cmake.in single-top-level-directory-rename logic, not
guessed): SOURCE_DIR = <CMAKE_CURRENT_BINARY_DIR>/m4-prefix/src/m4, so
COPYING lands there once the build step runs, before the top-level
install() rules execute. Added a second install(FILES ...) rule beside
the wrapper's existing "dummy README.md" one, pointing at that path -
patching the build to add the missing LICENSE, per
patching-and-licensing.md's documented remedy; will send the same fix
upstream to cmake-wheel/cmeel-m4.

No architectural riscv64 concern: GNU m4 is portable C with no
SIMD/arch-specific code path, already built on every Linux distro's
riscv64 port. Verified against the real published x86_64 wheel that the
actual binary is bin/m4 (plus a scripts/ entry point), no .so to link
cross-package.

docs/packages/cmeel-m4.yaml: license GPL-3.0-only; the built wheel now
carries GNU m4's own COPYING via the CMakeLists.txt patch above.