build: Move m4-only definitions to separate file.

build: Move m4-only definitions to separate file.

We currently pass a number of variables to m4 by specifying command
line options of the form -D var=val. This approach makes the output of
"make V=1" very verbose due to the three multi-line descriptions. To
addresses this issue, introduce defs.m4 which contains one m4 macro
definition for each former make variable. The end result should
be identical.

We tweak the M4 make variable to always read defs.m4 before the
first regular input file so that the suite and the web page see the
definitions. The third user of the former make variables is the README
phony target. To convert this as well, we rename the README file to
README.m4 and adjust the rule of the README phony target to run m4
on this file.

Switch to saner m4 quoting characters while at it.