Pull request testing should set -Werror

Created by: mhoemmen

@vbrunini asks whether pull request testing could set -Werror, so as to avoid issues like #3177 (closed).

@trilinos/framework @khpierson

Expectations

Trilinos -- at least the library, not necessarily tests and examples -- should build warning-free.

See #3177 (closed). There is an issue that it's impossible to fix warnings in some packages.

Sierra builds with warnings as errors, so they want Trilinos to build warning-free.

Exclude legacy packages like ML. Fix warnings. Add -Werror to at least one PR build.