build system modernization: (based on suggestion from Markus Elfring <Markus.Elfring*web.de> in bb #452) * use AC_CONFIG_HEADER, since AM_CONFIG_HEADER is obsolete * put configure files into auxiliary directory * fix main declaration in FD_SETSIZE test * check for failure on fopen in FD_SETSIZE test * move version from AM_INIT_AUTOMAKE to AC_INIT, old form was obsolete * eliminate automake warnings, update Makefile.am * rename .splitted to .split (requested by aCaB)