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.

CMakeLists.txt

@@ -363,6 +363,10 @@ option(WITH_LIBMV         "Enable Libmv structure from motion library" ON)
 option(WITH_LIBMV_SCHUR_SPECIALIZATIONS "Enable fixed-size schur specializations." OFF)
 mark_as_advanced(WITH_LIBMV_SCHUR_SPECIALIZATIONS)
 
+# Logging/unbit test libraries.
+option(WITH_SYSTEM_GFLAGS   "Use system-wide Gflags instead of a bundled one" OFF)
+mark_as_advanced(WITH_SYSTEM_GFLAGS)
+
 # Freestyle
 option(WITH_FREESTYLE     "Enable Freestyle (advanced edges rendering)" ON)
 
@@ -1317,15 +1321,27 @@ endif()
 # Configure GLog/GFlags
 
 if(WITH_LIBMV OR WITH_GTESTS OR (WITH_CYCLES AND WITH_CYCLES_LOGGING))
+	if(WITH_SYSTEM_GFLAGS)
+		find_package(Gflags)
+		if(NOT GFLAGS_FOUND)
+			message(FATAL_ERROR "System wide Gflags is requested but was not found")
+		endif()
+		# FindGflags does not define this, and we are not even sure what to use here.
+		set(GFLAGS_DEFINES)
+	else()
+		set(GFLAGS_DEFINES
+			-DGFLAGS_DLL_DEFINE_FLAG=
+			-DGFLAGS_DLL_DECLARE_FLAG=
+			-DGFLAGS_DLL_DECL=
+		)
+		set(GFLAGS_NAMESPACE "gflags")
+		set(GFLAGS_LIBRARIES extern_gflags)
+		set(GFLAGS_INCLUDE_DIRS "${PROJECT_SOURCE_DIR}/extern/gflags/src")
+	endif()
+
 	set(GLOG_DEFINES
 		-DGOOGLE_GLOG_DLL_DECL=
 	)
-
-	set(GFLAGS_DEFINES
-		-DGFLAGS_DLL_DEFINE_FLAG=
-		-DGFLAGS_DLL_DECLARE_FLAG=
-		-DGFLAGS_DLL_DECL=
-	)
 endif()
 
 #-----------------------------------------------------------------------------