Use glfags target

Update the usage of Google Flags (gflags) library the same way Glog
updated it [1]. This pushes the minimum required gflags version to
v2.2.0.

Remove the ceres specific define of CERES_GFLAGS_NAMESPACE and directly
use GFLAGS_NAMESPACE defined in gflags/gflags_declare.h [2].

In CeresConfig.cmake the hard coded paths for gflags are ommited.
Instead we rely on the gflagsConfig file to get closer to a relocatable
CeresConfig.cmake.
Furthermore use the find_dependency() [4] cmake function specifically
created for cmake-config files.

This change builds upon the explicit PUBLIC/PRIVATE link change [3].

[1] https://github.com/google/glog/pull/199
[2] https://github.com/gflags/gflags/blob/d9b184bd0026b16bb4c2fded75d56fb2cce50d66/src/gflags_declare.h.in#L43
[3] https://ceres-solver-review.googlesource.com/c/ceres-solver/+/16220
[4] https://cmake.org/cmake/help/latest/module/CMakeFindDependencyMacro.html

Change-Id: I9861a2699f2702bf1a5e99d07863a7e6639b7c39