[PATCH-for-5.0?] configure: Do not leave sphinx in-tree artifacts

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

When ./configure checks the sphinx version is new enough, it leaves
the docs/sphinx/__pycache__/ directory. Avoid this by using the '-B'
option (don't write .py[co] files on import) via the
PYTHONDONTWRITEBYTECODE environment variable.

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 configure | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/configure b/configure
index 22870f3867..ed524399c7 100755
--- a/configure
+++ b/configure
@@ -4936,7 +4936,7 @@ has_sphinx_build() {
     # sphinx-build doesn't exist at all or if it is too old.
     mkdir -p "$TMPDIR1/sphinx"
     touch "$TMPDIR1/sphinx/index.rst"
-    "$sphinx_build" -c "$source_path/docs" -b html "$TMPDIR1/sphinx" "$TMPDIR1/sphinx/out" >/dev/null 2>&1
+    PYTHONDONTWRITEBYTECODE=yes "$sphinx_build" -c "$source_path/docs" -b html "$TMPDIR1/sphinx" "$TMPDIR1/sphinx/out" >/dev/null 2>&1
 }
 
 # Check if tools are available to build documentation.
-- 
2.21.1