This also includes Bison version check. Versions before 3.0 don't
support them in a reliable way and we don't promise to work with
versions older than 2.4.
Make indentation and quotation consistent in configure macros.
Also remove --with-sysinclude option, which was broken for 7 years
and nobody complained.
Thanks to Ruben Kerkhof for source patches.
The patch allows to use autoreconf, replaces some long obsolete
constructs and does some other minor cleanups. Also, the file
configure.in is renamed to configure.ac, as the old name has been
deprecated for a long time.
Thanks to Ruben Kerkhof for the patchset.