Tool path handling on Windows

[egonelbre]

antlr-4.5.1-complete.jar doesn't behave consistently with "/" and "" on Windows.

Initial folder structure:

|   antlr-4.5.1-complete.jar
\---src
    \---main
        \---antlr
                example.g4

Case 1. After java -jar antlr-4.5.1-complete.jar -o src\main\generated src\main\antlr\example.g4:

|   antlr-4.5.1-complete.jar
\---src
    \---main
        +---antlr
        |       example.g4
        \---generated
            \---src
                \---main
                    \---antlr
                            example.tokens
                            exampleBaseListener.java
                            exampleLexer.java
                            exampleLexer.tokens
                            exampleListener.java
                            exampleParser.java

Case 2. After java -jar antlr-4.5.1-complete.jar -o src/main/generated src/main/antlr/example.g4:

|   antlr-4.5.1-complete.jar
\---src
    \---main
        +---antlr
        |       example.g4
        \---generated
                example.tokens
                exampleBaseListener.java
                exampleLexer.java
                exampleLexer.tokens
                exampleListener.java
                exampleParser.java

According to #683 the Case 1. behavior is correct, however in cross-platform scripts you are quite likely to use the path in Case 2.

Ideally both would behave exactly the same way.

[mike-lischke]

Also this one is likely a duplicate of #638.