Fix translation po file not found when `make rst`

The parsed language parameters contain unstripped spaces. This will
generate a wrong path.

Provide a `TOOLSOPT` to allow overriding the default values of
parameters of the `make_rst.py` script.

The xml generated by `godot --doctool -l LANG` can be checked for
errors using `make xml-check LANGARG=LANG`, which may be useful for
checking errors in po files.

doc/Makefile

@@ -4,7 +4,7 @@ OUTPUTDIR = $(BASEDIR)/_build
 TOOLSDIR = $(BASEDIR)/tools
 JSDIR = "$(BASEDIR)/../platform/web"
 LANGARG ?= en
-LANGCMD = -l $(LANGARG)
+TOOLSOPT ?=
 
 .ONESHELL:
 
@@ -19,7 +19,10 @@ doxygen:
 rst:
 	rm -rf "$(OUTPUTDIR)/rst"
 	mkdir -p "$(OUTPUTDIR)/rst"
-	python3 "$(TOOLSDIR)/make_rst.py" -o "$(OUTPUTDIR)/rst" "$(LANGCMD)" $(CLASSES)
+	python3 "$(TOOLSDIR)/make_rst.py" -o "$(OUTPUTDIR)/rst" -l "$(LANGARG)" $(TOOLSOPT) $(CLASSES)
+
+xml-check:
+	python3 "$(TOOLSDIR)/make_rst.py" --dry-run -l "$(LANGARG)" $(TOOLSOPT) $(CLASSES)
 
 rstjs:
 	rm -rf "$(OUTPUTDIR)/rstjs"