CMake: Add option to build against system-wide Glog

Similar to previous commit for Gflags.

CMakeLists.txt

@@ -365,7 +365,9 @@ 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)
+option(WITH_SYSTEM_GFLOG    "Use system-wide Glog instead of a bundled one" OFF)
 mark_as_advanced(WITH_SYSTEM_GFLAGS)
+mark_as_advanced(WITH_SYSTEM_GLOG)
 
 # Freestyle
 option(WITH_FREESTYLE     "Enable Freestyle (advanced edges rendering)" ON)
@@ -1339,9 +1341,24 @@ if(WITH_LIBMV OR WITH_GTESTS OR (WITH_CYCLES AND WITH_CYCLES_LOGGING))
 		set(GFLAGS_INCLUDE_DIRS "${PROJECT_SOURCE_DIR}/extern/gflags/src")
 	endif()
 
-	set(GLOG_DEFINES
-		-DGOOGLE_GLOG_DLL_DECL=
-	)
+	if(WITH_SYSTEM_GLOG)
+		find_package(Glog)
+		if(NOT GLOG_FOUND)
+			message(FATAL_ERROR "System wide Glog is requested but was not found")
+		endif()
+		# FindGlog does not define this, and we are not even sure what to use here.
+		set(GLOG_DEFINES)
+	else()
+		set(GLOG_DEFINES
+			-DGOOGLE_GLOG_DLL_DECL=
+		)
+		set(GLOG_LIBRARIES extern_glog)
+		if(WIN32)
+			set(GLOG_INCLUDE_DIRS ${CMAKE_SOURCE_DIR}/extern/glog/src/windows)
+		else()
+			set(GLOG_INCLUDE_DIRS ${CMAKE_SOURCE_DIR}/extern/glog/src)
+		endif()
+	endif()
 endif()
 
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