CMake: Add option to link against system-wide Gflags library

It is disabled by default, so should not affect existing configurations.

Main benefits of this goes as:

- Linux distros can use that to avoid libraries duplication and link
  blender package against gflags package from the system.

- It it easier to test whether Blender works with updated version of
  Gflags prior to re-bundling the library.

tests/gtests/CMakeLists.txt

@@ -6,6 +6,7 @@ if(WITH_GTESTS)
 
 	add_definitions(${GFLAGS_DEFINES})
 	add_definitions(${GLOG_DEFINES})
+	add_definitions(-DBLENDER_GFLAGS_NAMESPACE=${GFLAGS_NAMESPACE})
 
 	# Otherwise we get warnings here that we cant fix in external projects
 	remove_strict_flags()

tests/gtests/testing/CMakeLists.txt

@@ -24,7 +24,7 @@
 set(INC
 	.
 	..
-	../../../extern/gflags/src
+	${GFLAGS_INCLUDE_DIRS}
 	../../../extern/gtest/include
 )
 

tests/gtests/testing/testing_main.cc

@@ -28,7 +28,7 @@
 
 int main(int argc, char **argv) {
   testing::InitGoogleTest(&argc, argv);
-  gflags::ParseCommandLineFlags(&argc, &argv, true);
+  BLENDER_GFLAGS_NAMESPACE::ParseCommandLineFlags(&argc, &argv, true);
   google::InitGoogleLogging(argv[0]);
 
   return RUN_ALL_TESTS();