Issue when dealing with multiline variable.

For the following line, ninja report error `'inline_value1' doesn't match the target pattern.`

```
VAR := initvalue

VAR += \
some_value1 \
some_value2 \

VAR += inline_value1 inline_value2
```

Mixing multiline with single line will cause this issue. And if I put single line ahead, the problem gone.