[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.
diff --git a/configure.ac b/configure.ac
index 0b8724e..56fa55e 100755
--- a/configure.ac
+++ b/configure.ac
@@ -167,7 +167,6 @@
dnl -----------------
dnl System extensions
dnl -----------------
-AC_AIX
AC_GNU_SOURCE
dnl -------