make dynamically loadable module

ref: yaoweibin#234 (comment)

Author: pexcn

config

@@ -9,16 +9,20 @@ ngx_check_src="$ngx_addon_dir/ngx_http_upstream_check_module.c"
 ngx_feature_test="int a;"
 . auto/feature
 
-if [ $ngx_found = yes ]; then
+ngx_addon_name=ngx_http_upstream_check_module
+ngx_module_incs=$ngx_addon_dir
+
+if test -n "$ngx_module_link"; then
+    ngx_module_type="HTTP"
+    ngx_module_name="ngx_http_upstream_check_module"
+    ngx_module_srcs="$ngx_addon_dir/ngx_http_upstream_check_module.c"
+    ngx_module_deps="$ngx_addon_dir/ngx_http_upstream_check_module.h"
+
+    . auto/module
+else
     have=NGX_HTTP_UPSTREAM_CHECK . auto/have
     CORE_INCS="$CORE_INCS $ngx_feature_path"
-    ngx_addon_name=ngx_http_upstream_check_module
     HTTP_MODULES="$HTTP_MODULES ngx_http_upstream_check_module"
     NGX_ADDON_DEPS="$NGX_ADDON_DEPS $ngx_feature_deps"
     NGX_ADDON_SRCS="$NGX_ADDON_SRCS $ngx_check_src"
-else
-    cat << END
-    $0: error: the ngx_http_upstream_check_module addon error.
-END
-    exit 1
 fi