configure script failed on WSL

I ran configure script on WSL 1, but it failed as below:

```
$ bash configure --enable-debug --with-boot-jdk=/mnt/d/Java/jdk-15.0.1

    :

configure: Found potential Boot JDK using configure arguments
configure: The command for java_to_test, which resolves as "/mnt/d/Java/jdk-15.0.1/bin/java", can not be found.
configure: error: Cannot locate /mnt/d/Java/jdk-15.0.1/bin/java
configure exiting with result code 1
```

`fixpath.sh` attempts to run `$PATHTOOL` with `.exe` if it fails with original path, but `.exe` would not be added even if the path exists.