Set mac_deployment_target = "10.13.0"

Hopefully, this change will fix the following error:

../buildtools/third_party/libc++/trunk/src/barrier.cpp:37:43: error: aligned allocation function of type 'void *(std::size_t, std::align_val_t)' is only available on macOS 10.13 or newer
        __state = unique_ptr<__state_t[]>(new __state_t[__count]);
                                          ^

steps/05-configure.sh

@@ -24,7 +24,7 @@ mkdir -p "$BUILD"
       echo "ios_enable_code_signing = false"
       ;;
     mac)
-      echo 'mac_deployment_target = "10.11.0"'
+      echo 'mac_deployment_target = "10.13.0"'
       ;;
     win)
       echo 'pdf_use_win32_gdi = true'