Allow suppression of warnings from external dependencies

Summary:
Allow suppression of warnings from external dependencies

Also modify one STATUS message to avoid interpretation as a warning.
Both changes will avoid unnecessary warnings in LHCb builds.

Test Plan: Checked locally configure and compilation.

Reviewers: kreps

Reviewed By: kreps

Subscribers: #evtgen_admins

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D54