diff --git a/src/filter/levels/CMakeLists.txt b/src/filter/levels/CMakeLists.txt
index 081fcd87..13018f41 100644
--- a/src/filter/levels/CMakeLists.txt
+++ b/src/filter/levels/CMakeLists.txt
@@ -1,6 +1,6 @@
 # Set C99 flag for gcc
 if (CMAKE_COMPILER_IS_GNUCC)
-    set(CMAKE_C_FLAGS "-std=c99")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
 endif (CMAKE_COMPILER_IS_GNUCC)
 
 set (SOURCES levels.c)
diff --git a/src/filter/three_point_balance/CMakeLists.txt b/src/filter/three_point_balance/CMakeLists.txt
index b3e09f3a..c2e6a949 100644
--- a/src/filter/three_point_balance/CMakeLists.txt
+++ b/src/filter/three_point_balance/CMakeLists.txt
@@ -1,6 +1,6 @@
 # Set C99 flag for gcc
 if (CMAKE_COMPILER_IS_GNUCC)
-    set(CMAKE_C_FLAGS "-std=c99")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c99")
 endif (CMAKE_COMPILER_IS_GNUCC)
 
 set (SOURCES three_point_balance.c)