bpo-36588: On AIX, remove major version from sys.platform (GH-12787)

On AIX, sys.platform doesn't contain the major version anymore.
Always return 'aix', instead of 'aix3' .. 'aix7'.  Since
older Python versions include the version number, it is recommended to
always use sys.platform.startswith('aix').
diff --git a/configure b/configure
index fcfa714..72589fd 100755
--- a/configure
+++ b/configure
@@ -3281,6 +3281,7 @@
     MACHDEP="$ac_md_system$ac_md_release"
 
     case $MACHDEP in
+	aix*) MACHDEP="aix";;
 	linux*) MACHDEP="linux";;
 	cygwin*) MACHDEP="cygwin";;
 	darwin*) MACHDEP="darwin";;
@@ -10199,7 +10200,6 @@
 
 
 
-
 if test "x$ac_cv_env_PKG_CONFIG_set" != "xset"; then
 	if test -n "$ac_tool_prefix"; then
   # Extract the first word of "${ac_tool_prefix}pkg-config", so it can be a program name with args.