Drop autogen.sh: Do use autoconf

Automake is changing and some of the stuff in autogen.sh is expected to fail
soon:
https://www.gnu.org/software/automake/manual/html_node/Future-of-aclocal.html

"autoreconf -if" will overwrite the INSTALL file, so its content is now in the
README file.

Added NEWS warning

Adjusted .gitignore