Fixes osrf/docker_images#403.

We use Connext Module with two different purposes:

• Just check if Connext is available (e.g. here).
• Be able to compile/link against connext.

In the first case, variables like Connext_GLIBCXX_USE_CXX11_ABI_ZERO aren't of interest.

This PR does two changes:

• abbe78b modifies the example project to only require CXX language. That already fixes Cmake issues with osrf/ros2:nightly-rmw-nonfree image tag osrf/docker_images#403.
• e53473a goes a bit further and avoids trying to set Connext_GLIBCXX_USE_CXX11_ABI_ZERO if the project that find-pkged Connext Module doesn't have CXX language support (in that case, setting Connext_GLIBCXX_USE_CXX11_ABI_ZERO is for sure not of interest).