Replace use of GFLAGS_LIBRARIES with export gflags target

- As our minimum required version of gflags (2.2) exports itself as
  a CMake package and this is the case for the default 18.04 package
  we can use the gflags target directly.
- Replaces forced use of CONFIG in find_package(gflags) with a check
  that the gflags imported target exists to avoid ambiguity with
  libgflags if installed in a default location.  This permits users to
  override the gflags detection should they so choose, provided that
  they do so via an imported target.
- Also removes some previously removed legacy GLAGS_ vars from the
  installation docs.

Change-Id: I015f5a751e5b22f956bbf9df692e63a6825c9f0d