Currently, the PhutilLibraryMapBuilder will fail in an obscure way if the XHPAST binary is not built. The issue is that phutil_symbols.php will attempt to build the XHPAST binary, and this will happen eight times concurrently. Ultimately, this will fail with:
<libphutil>/scripts/build_xhpast.sh: line 24: ./xhpast: Text file busy