Make constructorExists take in mind customized generic builders

[ay0ks]

Fixes #3935

[ay0ks]

I was able to build the project (missed ant dist the first time) and can confirm that the issue is resolved. Example code provided in #3935 (comment) compiles and runs with little changes.

It would be better to make generated default value providers protected to be able to be used inside the customized builder constructor.

[janrieke]

Thanks for your contribution. I'll have a look at the PR next week. Until then, please:

• add your name to the AUTHORS file,
• verify that Eclipse is not affected, and
• add a test case or extend an existing ("transform" folder).