In a minimal build environment (no sphinx), the package cannot initially
clean itself because the upstream doc/Makefile checks for sphinx-build
executable and Debian doesn't get any benefit of that.
Thus do not check for the presence of sphinx-build when running the
clean or help target by adding a phony check_sphinx-build that does the
check for sphinx-build.
Debian bug: https://bugs.debian.org/851559
Signed-off-by: Benjamin Drung <benjamin.drung@profitbricks.com>
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