Gradle script shouldn't use org.gradle.internal.* classes

[swpalmer]

https://github.com/johanvos/javafx11samples/blob/2e9183d36658947d526a5ff21763d2f626d71975/topics/javafx3d/hello3d/build.gradle#L8

Consider instead:

def os =System.getProperty('os.name').toLowerCase()
if (os.contains('windows')) {
...
} else if (os.contains('linux')) {
...
} else if (os.contains('mac')) {
...
}

[mkroening]

This already is a discussion on this: https://github.com/openjfx/openjfx-docs/issues/6