make: Add target to check generated/derived files were checked in

A new target `check-gen-updated` to verify that all derived/generated
files that were modified were also checked in. This is used on CI to
check, and is not added to `check` since it'll complain on dirty
trees, i.e., before the devs check in their changes.

Suggested-by: Rusty Russell <@rustyrussell>

.github/workflows/ci.yaml

@@ -16,9 +16,9 @@ jobs:
         include:
           # CFG us just an enumeration so we can refer to the configurations more easily
           - {CFG:  1, TEST_CMD: "make", NO_PYTHON: 1}
-          - {CFG:  2, TEST_CMD: "make check-source check-units installcheck", COPTFLAGS: "-O3"}
+          - {CFG:  2, TEST_CMD: "make check-source check-units installcheck check-gen-updated", COPTFLAGS: "-O3"}
           - {CFG:  3, ARCH: 32, TEST_CMD: "make check-source check-units installcheck", DEVELOPER: 0}
-          - {CFG:  4, TEST_CMD: "make check-source check-units installcheck", EXPERIMENTAL_FEATURES: 1}
+          - {CFG:  4, TEST_CMD: "make check-source check-units installcheck check-gen-updated", EXPERIMENTAL_FEATURES: 1}
     steps:
       - name: Checkout
         uses: actions/checkout@v2.0.0

Makefile

@@ -485,6 +485,18 @@ check-source: check-makefile check-source-bolt check-whitespace check-markdown c
 
 full-check: check check-source
 
+# Simple target to be used on CI systems to check that all the derived
+# files were checked in and updated. It depends on the generated
+# targets, and checks if any of the tracked files changed. If they did
+# then one of the gen-targets caused this change, meaning either the
+# gen-target is not reproducible or the files were forgotten.
+#
+# Do not run on your development tree since it will complain if you
+# have a dirty tree.
+check-gen-updated: $(ALL_GEN_HEADERS) $(ALL_GEN_SOURCES) wallet/statements_gettextgen.po $(MANPAGES)
+	@echo "Checking for generated files being changed by make"
+	git diff --exit-code HEAD $?
+
 coverage/coverage.info: check pytest
 	mkdir coverage || true
 	lcov --capture --directory . --output-file coverage/coverage.info