Error on implicit declaration

CMakeLists.txt

@@ -31,9 +31,9 @@ if(${CMAKE_C_COMPILER_ID} MATCHES "Clang" OR ${CMAKE_CXX_COMPILER_ID} MATCHES "C
 endif()
 
 if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX OR CMAKE_COMPILER_IS_CLANG)
-	set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wno-unused-function ${CMAKE_CXX_FLAGS}")
-	set(CMAKE_C_FLAGS "-Wall -Wextra -Wno-unused-function -Wno-missing-field-initializers ${CMAKE_C_FLAGS} -std=gnu99")
-	
+	set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wno-unused-function -Werror-implicit-function-declaration -Wno-missing-field-initializers ${CMAKE_CXX_FLAGS}")
+	set(CMAKE_C_FLAGS "-Wall -Wextra -Wno-unused-function -Werror-implicit-function-declaration ${CMAKE_C_FLAGS} -std=gnu99")
+
 	option(USE_LIBC++ "Use libc++ instead of libstdc++" ${APPLE})
 	if(USE_LIBC++)
 		set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -stdlib=libc++")