Add examples to the Bazel build

Also moves the gflags namespace define
into the ceres_library() target.

Change-Id: Ibb8f3fdd9a8c371fe96b475f752f8320ffe63cdc

bazel/ceres.bzl

@@ -130,7 +130,9 @@ CERES_SRCS = ["internal/ceres/" + filename for filename in [
 # TODO(rodrigoq): add support to configure Ceres into various permutations,
 # 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):
+def ceres_library(name,
+                  restrict_schur_specializations=False,
+                  gflags_namespace="gflags"):
     # The path to internal/ depends on whether Ceres is the main workspace or
     # an external repository.
     if native.repository_name() != '@':
@@ -188,6 +190,7 @@ def ceres_library(name, restrict_schur_specializations=False):
             "CERES_NO_THREADS",
             "CERES_NO_LAPACK",
             "CERES_STD_UNORDERED_MAP",
+            "CERES_GFLAGS_NAMESPACE=" + gflags_namespace,
         ],
         includes = [
             "config",