Add stricter ant/build option for developers? [LUCENE-6128]

Can we add a build option (-Dpendantic=true) that is more anal?

E.g. on backporting a change from trunk (java 8) to 5.x (java 7) I very rarely remember to "use java 7" to compile/test. Sure, we set javac.source/target to 1.7, but that's not perfect (e.g. an interface w/ a default method as of java 8).

With this option set we could do things like require you are using java 7 on 5.x, fail on warnings, etc.

Casual users/developers can then still use java 8 to run tests, etc. (the option would default to off).

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Migrated from [LUCENE-6128](https://issues.apache.org/jira/browse/LUCENE-6128) by Michael McCandless (@mikemccand)

Start by reviewing the Ant build configuration and the existing javac.source/target settings for the 5.x line. Determine how an opt-in strict mode could enforce Java 7 usage and fail on warnings while leaving the default behavior unchanged; done means the requested developer checks are defined and covered by build validation.

Written by the indexing model from the issue text.