diff --git a/CMakeLists.txt b/CMakeLists.txt
index cb3743fd..45a3a2b6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -11,8 +11,8 @@ set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/cmake/")
 
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY bin)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY lib)
-set(CMAKE_C_FLAGS "-O2 -Wall -Wextra")
-set(CMAKE_C_FLAGS_DEBUG "-O0 -g -DDEBUG=1")
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -O2 -Wall -Wextra")
+set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -O0 -g -DDEBUG=1")
 
 option(STRICT_C89_MODE "Strict c89 mode" OFF)
 if(STRICT_C89_MODE)