make: add filter dependencies to html recipe

Makefile

@@ -4,6 +4,9 @@ EXCLUDES = README.md LAYOUT.md FAQ.md DESIGN.md
 SRC_PAGES = $(filter-out $(EXCLUDES), $(MARKDOWN))
 DST_PAGES = $(patsubst %.md,%.html,$(SRC_PAGES))
 
+# Pandoc filters
+FILTERS = $(wildcard tools/filters/*.py)
+
 # Inclusions.
 INCLUDES = \
 	-Vheader="$$(cat _includes/header.html)" \
@@ -24,7 +27,7 @@ motivation.html : motivation.md _layouts/slides.html
 	-o $@ $<
 
 # Pattern to build a generic page.
-%.html : %.md _layouts/page.html
+%.html : %.md _layouts/page.html $(FILTERS)
 	pandoc -s -t html \
 	--template=_layouts/page \
 	--filter=tools/filters/blockquote2div.py \