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

bazel/ceres.bzl

@@ -137,8 +137,7 @@ CERES_SRCS = ["internal/ceres/" + filename for filename in [
 # like SuiteSparse or not, threading or not, glog or not, and so on.
 # See https://github.com/ceres-solver/ceres-solver/issues/335.
 def ceres_library(name,
-                  restrict_schur_specializations=False,
-                  gflags_namespace="gflags"):
+                  restrict_schur_specializations=False):
     # The path to internal/ depends on whether Ceres is the main workspace or
     # an external repository.
     if native.repository_name() != '@':
@@ -198,7 +197,6 @@ def ceres_library(name,
             "CERES_NO_LAPACK",
             "CERES_USE_EIGEN_SPARSE",
             "CERES_USE_CXX11_THREADS",
-            "CERES_GFLAGS_NAMESPACE=" + gflags_namespace,
         ],
         includes = [
             "config",