autoconf: Some updates

* AC_HELP_STRING -> AS_HELP_STRING
* AC_AIX, AC_MINIX -> AC_USE_SYSTEM_EXTENSIONS
* AC_ISC_POSIX -> AC_SEARCH_LIBS([strerror],[cposix])
* AC_PROG_LD -> LT_PATH_LD
* AC_OUTPUT -> AC_CONFIG_FILES + AC_OUTPUT