configure: redirect sphinx-build check to config.log

Otherwise it's hard to debug whats going on.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-Id: <20200414200631.12799-8-alex.bennee@linaro.org>

configure

@@ -4942,7 +4942,9 @@ 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" $sphinx_werror -c "$source_path/docs" -b html "$TMPDIR1/sphinx" "$TMPDIR1/sphinx/out" >/dev/null 2>&1
+    "$sphinx_build" $sphinx_werror -c "$source_path/docs" \
+                    -b html "$TMPDIR1/sphinx" \
+                    "$TMPDIR1/sphinx/out"  >> config.log 2>&1
 }
 
 # Check if tools are available to build documentation.