Install and use find-module for GFlags

[jamiesnape]

Relates in part to RobotLocomotion/drake-external-examples#145. Basically the find-module from drake-external-examples.

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This change is 

[jamiesnape]

+@BetsyMcPhail for feature review.

[BetsyMcPhail]

[jamiesnape]

+@ggould-tri for platform review.

[jwnimmer-tri]

I don't necessarily think it should prevent us from merging this PR, but FYI I've been thinking we should stop installing text_logging_gflags.h, in order to avoid the complexity (both what's already there, and this new stuff, and the next new stuff we'll need again later) vs the low benefit. Instead, I would propose adding two string constants for the default value and help to text_logging.h, and let the users do the DEFINE and FLAGS call themselves.

[ggould-tri]

I agree with @jwnimmer-tri both that we could and probably should simplify log setup going forward, and that this PR is fine for now regardless.

I wonder how many users would be affected.

Reviewed 3 of 3 files at r1.
Reviewable status: complete! all discussions resolved, LGTM from assignees ggould-tri(platform),BetsyMcPhail (waiting on @BetsyMcPhail)