configure: Pick sphinx-build-3 when available

The next commit will require a sphinx-build that uses Python 3.  On
some systems, sphinx-build is fine, on others you need to use
sphinx-build-3.  To keep things working out of the box on both kinds
of systems, try sphinx-build-3, then sphinx-build.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <87a75lqe8e.fsf@dusky.pond.sub.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

configure

@@ -584,7 +584,6 @@ query_pkg_config() {
 }
 pkg_config=query_pkg_config
 sdl2_config="${SDL2_CONFIG-${cross_prefix}sdl2-config}"
-sphinx_build=sphinx-build
 
 # If the user hasn't specified ARFLAGS, default to 'rv', just as make does.
 ARFLAGS="${ARFLAGS-rv}"
@@ -915,6 +914,17 @@ do
         break
     fi
 done
+
+sphinx_build=
+for binary in sphinx-build-3 sphinx-build
+do
+    if has "$binary"
+    then
+        sphinx_build=$(command -v "$binary")
+        break
+    fi
+done
+
 : ${smbd=${SMBD-/usr/sbin/smbd}}
 
 # Default objcc to clang if available, otherwise use CC
@@ -4803,7 +4813,7 @@ 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
+    "$sphinx_build" -c "$source_path/docs" -b html "$TMPDIR1/sphinx" "$TMPDIR1/sphinx/out" >/dev/null 2>&1
 }
 
 # Check if tools are available to build documentation.