config: separate make target for diagrams

Separate the build step and commit the output to the repository. This way the docs can be built on platforms that do not have inkscape, such as readthedocs.

pipenv run make -C doc diagrams
pipenv run make -C doc html

doc/Makefile

@@ -14,8 +14,11 @@ help:
 
 .PHONY: help Makefile
 
+diagrams:
+	@$(MAKE) -C "$(SOURCEDIR)"/diagram_source clean
+	@$(MAKE) -C "$(SOURCEDIR)"/diagram_source
+
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(MAKE) -C "$(SOURCEDIR)"/diagram_source `[ "$@" = "clean" ] && echo clean || echo all`
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)

doc/api/diagram_source/Makefile

@@ -4,13 +4,13 @@ OUT_DIR = ../images
 
 POSTFIX := _label
 SVG := $(wildcard $(SRC_DIR)/*.svg)
-PNG := $(patsubst $(SRC_DIR)/%.svg,$(OUT_DIR)/%$(POSTFIX).png,$(SVG))
+DIAGRAMS := $(patsubst $(SRC_DIR)/%.svg,$(OUT_DIR)/%$(POSTFIX).png,$(SVG))
 DIR := ${CURDIR}
 
-all: $(PNG)
+all: $(DIAGRAMS)
 
 clean:
-	rm -f $(PNG)
+	rm -f $(DIAGRAMS)
 
 $(OUT_DIR)/%$(POSTFIX).png: $(SRC_DIR)/%.svg
 	inkscape --file=$(DIR)/$< --export-area-drawing --export-png=$(DIR)/$@

doc/api/images/.gitignore

@@ -1 +0,0 @@
-*_label.png