Serialize latex prerequisites before running build

Adding _ensure-sphinxcontrib-svg2pdfconverter as a sibling prerequisite of build makes make -jN latex nondeterministic, because GNU Make may run both prerequisites in parallel. In that mode, build can start before the converter package (and even the venv it depends on) is installed, so the LaTeX doc build can fail intermittently with missing tooling even though the target is supposed to prepare it first.

Author: m-aciek

Doc/Makefile

@@ -88,9 +88,9 @@ htmlhelp: build
 	      "build/htmlhelp/pydoc.hhp project file."
 
 .PHONY: latex
-latex: _ensure-sphinxcontrib-svg2pdfconverter
 latex: BUILDER = latex
-latex: build
+latex: _ensure-sphinxcontrib-svg2pdfconverter
+	$(MAKE) build BUILDER=$(BUILDER)
 	@echo "Build finished; the LaTeX files are in build/latex."
 	@echo "Run \`make all-pdf' or \`make all-ps' in that directory to" \
 	      "run these through (pdf)latex."