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.
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