Doc/Makefile: fix race condition in latex target

When running `make -j`, the `_ensure-sphinxcontrib-svg2pdfconverter` and `build` prerequisites of the `latex` target could execute in parallel. If `build` started before the venv was created by `_ensure-sphinxcontrib-svg2pdfconverter`, `sphinx-build` would not be found and the build would fail.

Fix this by removing `build` from the prerequisites and invoking it explicitly from the recipe, ensuring the venv is fully set up before the Sphinx build begins.

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."