Use a m4 helper macro to build plugin lists

m4/macros/add-plugin.m4

@@ -0,0 +1,10 @@
+# ADD_PLUGIN(plugin, category list)
+# -----------------------------------
+# Append the plugin name $1 to the category list variable $2_plugin
+AC_DEFUN([ADD_PLUGIN],
+	if test [patsubst(x$$1, [-], [_])] = xtrue; then
+		[m4_foreach_w([category], [$2],
+			[m4_format([%s_plugins=${%s_plugins}" $1"], category, category)]
+		)]
+	fi
+)