bugfix: config: avoided an error with NGINX 1.17.0 and above.

The postpone filter is unconditionally enabled with NGINX 1.17.0 and
above.

See:

nginx/nginx@a39380a

Author: thibaultcha

config

@@ -30,21 +30,24 @@ ECHO_DEPS="                                                                 \
         $ngx_addon_dir/src/ngx_http_echo_foreach.h                          \
         "
 
-# nginx won't have HTTP_POSTPONE_FILTER_MODULE & HTTP_POSTPONE_FILTER_SRCS
-# defined since 1.9.11
-if test -z "$HTTP_POSTPONE_FILTER_MODULE"; then
-    HTTP_POSTPONE_FILTER_MODULE=ngx_http_postpone_filter_module
-    HTTP_POSTPONE_FILTER_SRCS=src/http/ngx_http_postpone_filter_module.c
-fi
+# nginx 1.17.0+ unconditionally enables the postpone filter
+if [ ! -z "$HTTP_POSTPONE" ]; then
+    # nginx won't have HTTP_POSTPONE_FILTER_MODULE & HTTP_POSTPONE_FILTER_SRCS
+    # defined since 1.9.11
+    if [ -z "$HTTP_POSTPONE_FILTER_MODULE" ]; then
+        HTTP_POSTPONE_FILTER_MODULE=ngx_http_postpone_filter_module
+        HTTP_POSTPONE_FILTER_SRCS=src/http/ngx_http_postpone_filter_module.c
+    fi
 
-# This module depends upon the postpone filter being activated
-if [ $HTTP_POSTPONE != YES ]; then
-    HTTP_FILTER_MODULES="$HTTP_FILTER_MODULES $HTTP_POSTPONE_FILTER_MODULE"
-    HTTP_SRCS="$HTTP_SRCS $HTTP_POSTPONE_FILTER_SRCS"
-    HTTP_POSTPONE=YES
+    # This module depends upon the postpone filter being activated
+    if [ $HTTP_POSTPONE != YES ]; then
+        HTTP_FILTER_MODULES="$HTTP_FILTER_MODULES $HTTP_POSTPONE_FILTER_MODULE"
+        HTTP_SRCS="$HTTP_SRCS $HTTP_POSTPONE_FILTER_SRCS"
+        HTTP_POSTPONE=YES
+    fi
 fi
 
-if test -n "$ngx_module_link"; then
+if [ -n "$ngx_module_link" ]; then
     ngx_module_type=HTTP_AUX_FILTER
     ngx_module_name=$ngx_addon_name
     ngx_module_incs=