INPYTHON-569 Fix handling of cmake version (#292)

Author: blink1073

bindings/python/build-libbson.sh

@@ -45,6 +45,7 @@ pushd "$WORKDIR"
     then
       cmake -DENABLE_AUTOMATIC_INIT_AND_CLEANUP=OFF \
             -DENABLE_MONGOC=OFF \
+            -DCMAKE_POLICY_VERSION_MINIMUM=3.5 \
             -DCMAKE_OSX_ARCHITECTURES=${CMAKE_OSX_ARCHITECTURES} \
             -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE} \
             -DCMAKE_OSX_DEPLOYMENT_TARGET=${MACOSX_DEPLOYMENT_TARGET} \