m4: Upgrade to 1.4.20 release

m4: Upgrade to 1.4.20 release

This has been 4 years in making, it has fixes to build with gcc-15
and some patches are upstream [1] [2]

This release collects several years of portability improvements, as
well as a couple of minor optimizations to performance.  Notable
improvements in this release include faster execution of the 'eval'
builtin. More details [3]

[1] https://github.com/coreutils/gnulib/commit/2d830e4a792fcd9f614ed08a7f18584b8b21d23b
[2] https://git.savannah.gnu.org/gitweb/?p=libsigsegv.git;a=commitdiff;h=a6ff69873110c0a8ba6f7fd90532dbc11224828c
[3] https://lists.gnu.org/archive/html/m4-announce/2025-05/msg00000.html

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>