cmake: add more hardening compiler flags

In order to spot possible issues with direct impact on security during
QA on CI (GCC version 6 and higher).

Ref: https://developers.redhat.com/blog/2018/03/21/compiler-and-linker-flags-gcc/
Signed-off-by: Petr Štetiar <ynezz@true.cz>

CMakeLists.txt

@@ -6,7 +6,8 @@ PROJECT(ubox C)
 
 ADD_DEFINITIONS(-Wall -Werror)
 IF(CMAKE_C_COMPILER_VERSION VERSION_GREATER 6)
-	ADD_DEFINITIONS(-Wextra)
+	ADD_DEFINITIONS(-Wextra -Werror=implicit-function-declaration)
+	ADD_DEFINITIONS(-Wformat -Werror=format-security -Werror=format-nonliteral)
 ENDIF()
 ADD_DEFINITIONS(-Os -std=gnu99 -g3 -Wmissing-declarations -Wno-unused-parameter)