doc: Replace recursive Makefile with inclusion

Always rebuild the documentation directory. The speed benefit is not
worth the added complexity.

Makefile

@@ -77,7 +77,6 @@ clean:
 	@rm -rf $(OBJECTDIR)
 	@rm -rf $(LIBDIR)
 	@rm -rf $(COVDIR)
-	$(MAKE) -C doc clean
 
 dist:
 	@echo "Creating release tarball $(TARBALL) ... "
@@ -125,7 +124,6 @@ prof:
 		--callgrind-out-file=$(OBJECTDIR)/callgrind.out.%p $(OBJECTDIR)/profiler
 	callgrind_annotate $(OBJECTDIR)/callgrind.* > $(OBJECTDIR)/profile.txt
 
-doc:
-	$(MAKE) -C doc
+.PHONY: cov
 
-.PHONY: cov doc
+include doc/doc.mk

doc/Makefile

@@ -1,25 +0,0 @@
-DESTDIR ?= html
-
-all: $(DESTDIR)/index.html
-
-$(DESTDIR)/alog-arch.svg: arch/alog-arch.svg
-	scour -i $< -o $@ \
-	  --enable-comment-stripping \
-	  --enable-id-stripping \
-	  --enable-viewboxing \
-	  --indent=none \
-	  --shorten-ids
-
-$(DESTDIR)/index.html: $(DESTDIR)/fonts
-$(DESTDIR)/index.html: $(DESTDIR)/alog-arch.svg
-$(DESTDIR)/index.html: index ../README css/styles.css | $(DESTDIR)
-	asciidoctor $< -o $@
-
-$(DESTDIR)/fonts: fonts | $(DESTDIR)
-	cp -R $< $@
-
-$(DESTDIR):
-	mkdir -p $@
-
-clean:
-	rm -rf $(DESTDIR)

doc/doc.mk

@@ -0,0 +1,21 @@
+# Makefile snippet intended for inclusion by ../Makefile.
+
+build-doc:
+	rm -fr doc/html
+	mkdir doc/html
+
+	scour -i doc/arch/alog-arch.svg \
+	  -o doc/html/alog-arch.svg \
+	  --enable-comment-stripping \
+	  --enable-id-stripping \
+	  --enable-viewboxing \
+	  --indent=none \
+	  --shorten-ids
+
+	cp -a --reflink=auto doc/fonts doc/html
+
+	asciidoctor doc/index -o doc/html/index.html
+
+clean: clean-doc
+clean-doc:
+	rm -fr doc/html