Build: configure: move version.m4 to m4 subdirectory

... for consistency and to de-clutter the top-level directory.

With this and other recent changes, anyone who wants to rebuild in an existing
checkout must first run (once):

rm -rf aclocal.m4 autom4te.cache config.guess config.log config.status \
       config.sub configure depcomp install-sh libltdl libtool ltmain.sh \
       missing py-compile test-driver
./autogen.sh