Upstream ships codemirror as prebuilt.
Since it's removed in debian sources, we have to rebuild it on compile time
--- a/src/editor/libs/Makefile-codemirror
+++ b/src/editor/libs/Makefile-codemirror
@@ -6,7 +6,7 @@
 
 all:
 	mkdir -p "$(DESTDIR)"
-	cd codemirror && cp -r addon keymap lib mode theme LICENSE "$(DESTDIR)"/
+	cd codemirror && rollup -c && cp -r addon keymap lib mode theme LICENSE "$(DESTDIR)"/
 	# Clean unnecessary files within "mode"
 	find "$(DESTDIR)"/mode/ -type f -name '*.html' -exec rm -f {} +
 	find "$(DESTDIR)"/mode/ -type f -name 'test.js' -exec rm -f {} +
