build: Move description text to separate file.

build: Move description text to separate file.

The three description texts are passed to m4 via -D var=val command
line options, and they are output by the README target. The C
preprocessor does not need them, though.

This patch renames the README file to README.m4, moves the part of the
Makefile which defines the README variable to README.m4 and adjusts
the README rule to run m4 on README.m4 to expand the descriptions.

As as result, the three -D descriptionX options to m4 are now the
only place where the corresponding make variables are used. Since
the values consist of multiple lines, the verbose make output looks
rather messy, so create defs.m4 and include this file before the real
input file is read.

Switch to sane m4 quoting characters while at it.