diff --git a/.gitignore b/.gitignore
new file mode 100644
index 000000000..e7b97c5bd
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+.depends
diff --git a/GNUmakefile b/GNUmakefile
index 22b1a65b7..0335447ed 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -12,10 +12,13 @@ check: expected output
 
 output: $(OUTPUTS)
 
-output/%.txt: demos/%.lmp
+output/%.txt: demos/%.lmp $(SOURCE_PORT)
 	@mkdir -p $(dir $@)
 	./testrunner $< $@
 
+.depends: makedepends
+	./makedepends $@
+
 extract/%:
 	unzip $(UNZIPOPTS) -d extract $< $(notdir $@)
 
@@ -29,3 +32,6 @@ extract/class_ep.wad: pwads/class_ep.zip
 clean:
 	rm -f extract/*.wad
 	rm -rf output/*
+	rm -f .depends
+
+include .depends
diff --git a/makedepends b/makedepends
new file mode 100755
index 000000000..03c287ef7
--- /dev/null
+++ b/makedepends
@@ -0,0 +1,25 @@
+#!/usr/bin/env python3
+
+from glob import glob
+import os
+import sys
+
+from common import *
+
+depends_file = sys.argv[1]
+dependencies = {}
+
+for filename in glob("expected/**/*.txt", recursive=True):
+    lmp_file = filename.replace("expected/", "demos/").replace(".txt", ".lmp")
+    cfg = read_config(lmp_file)
+
+    pwad = cfg.get("pwad")
+    if pwad is not None:
+        out_file = filename.replace("expected/", "output/")
+        dependencies[out_file] = [os.path.join("extract", pwad)]
+
+with open(depends_file, "w") as f:
+    for out_file, depends in sorted(dependencies.items()):
+        f.write("%s: %s\n" % (
+            out_file, " ".join(depends),
+        ))