diff --git a/CMakeLists.txt b/CMakeLists.txt
index c3ab49da8db..3010147876b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -375,7 +375,7 @@ SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")  # FIXME: should be -std=
 
 
 IF (UNIX)
-  SET( CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror -Wdeclaration-after-statement -Wno-pointer-sign -D_FORTIFY_SOURCE=2 -Wall -Wunused -Wmissing-prototypes -Wmissing-declarations  -Wno-unused")
+  SET( CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror -Wdeclaration-after-statement -Wno-pointer-sign -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -Wall -Wunused -Wmissing-prototypes -Wmissing-declarations  -Wno-unused")
   SET( CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-error=deprecated-declarations -std=gnu99 -O2")
 ENDIF (UNIX)
 IF (MINGW)