fix warning output directory not created

create_documentation

@@ -131,6 +131,8 @@
     if [[ -d "$doc_dir" ]] ; then
         pushd $doc_dir
         for language in $languages ; do
+            # create _build to output warnings into it
+            [[ -d _build ]] || mkdir _build/
             make $format LANGUAGE=${language} BUILDDIR=_build SPHINXOPTS="-w _build/warnings.out"
             if [[ $? -eq 0 ]] ; then
                 dest_dir=$out_dir/$project_name/$language/$branch