diff --git a/extern/CMakeLists.txt b/extern/CMakeLists.txt
index 35da67d05a4..1c8ae6bd5df 100644
--- a/extern/CMakeLists.txt
+++ b/extern/CMakeLists.txt
@@ -70,7 +70,9 @@ if(WITH_LIBMV OR WITH_GTESTS OR (WITH_CYCLES AND WITH_CYCLES_LOGGING))
   if(NOT WITH_SYSTEM_GFLAGS)
     add_subdirectory(gflags)
   endif()
-  add_subdirectory(glog)
+  if(NOT WITH_SYSTEM_GLOG)
+    add_subdirectory(glog)
+  endif()
 endif()
 
 if(WITH_GTESTS)
diff --git a/extern/ceres/CMakeLists.txt b/extern/ceres/CMakeLists.txt
index b723a4466fb..58f64f23fd8 100644
--- a/extern/ceres/CMakeLists.txt
+++ b/extern/ceres/CMakeLists.txt
@@ -8,6 +8,12 @@ set(INC
   config
 )
 
+if(NOT WITH_SYSTEM_GFLAGS)
+  list(APPEND INC
+    ../gflags/src
+  )
+endif()
+
 set(INC_SYS
   ${EIGEN3_INCLUDE_DIRS}
   ${GFLAGS_INCLUDE_DIRS}
diff --git a/extern/glog/CMakeLists.txt b/extern/glog/CMakeLists.txt
index f25c72d830e..bb0dd29d153 100644
--- a/extern/glog/CMakeLists.txt
+++ b/extern/glog/CMakeLists.txt
@@ -3,7 +3,6 @@
 
 set(INC
   src
-  ../gflags/src
 )
 
 set(INC_SYS
@@ -46,6 +45,9 @@ set(LIB
 )
 
 if(NOT WITH_SYSTEM_GFLAGS)
+  list(APPEND INC
+    ../gflags/src
+  )
   list(APPEND LIB
     extern_gflags
   )