RFE: add md5sum as a built-in

[Bruno Haible]

Hi,

It would be useful to have md5sum as a built-in macro in m4.
For example,
  md5sum(`frankie goes to hollywood') => 5b8a258083161281a1ad2c25d1505856

The first use of this function would be to map file names to shell variables
in a way similar to AS_TR_SH of autoconf, except that it would not be
subject to collisions:
  AS_TR_SH([sys/file.h]) and AS_TR_SH([sys_file.h]) collide.

The second use would be for content based addressing in general, in the same
way as 'git' is based on a one-way hash function at the lowest level.

In theory it would be possible to use the 'esyscmd' built-in. But that's
likely too slow when you use the function in a number of places. (For
my first use-case, there would be 300 invocations when autoconf is asked
to process a particular, real-life configure.ac file.)

Bruno