[configure] remote support for AIX, it conflicts on newer autoconf

AC_AIX and AC_GNU_SOURCE conflict > autoconf 2.53 or so,
but the replacement AC_USE_SYSTEM_EXTENSIONS isn't available
until later versions.  So either we narrow the supported
autconf version range, or disable AIX (if it has any effect),
or determin that AC_AIX doesn't do anything useful anyhow.
1 file changed