Log Message:
m4: update to 1.4.19.

* Noteworthy changes in release 1.4.19 (2021-05-28) [stable]

** A number of portability improvements inherited from gnulib, including
   the ability to perform stack overflow detection on more platforms
   without linking to GNU libsigsegv.

* Noteworthy changes in release 1.4.18d (2021-05-11) [beta]

** A number of portability improvements inherited from gnulib.

* Noteworthy changes in release 1.4.18b (2021-05-07) [beta]

** The symbol hash table now defaults to 65537 buckets instead of 509, as
   modern systems have enough memory to benefit from fewer hash collisions
   by default.

** Introduce the use of gettext, with the immediate benefit of nicer
   UTF-8 author names.  Over time, more translations of program messages
   will become available.

** A number of portability improvements inherited from gnulib.