CMake: Use CMAKE_C_FLAGS instead of add_compile_options

This is for compatibility reasons with old CMake 2.8 versions.

CMakeLists.txt

@@ -12,7 +12,7 @@ set(CJSON_UTILS_VERSION_SO 1)
 set(PROJECT_VERSION "${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}.${PROJECT_VERSION_PATCH}")
 
 if(("${CMAKE_C_COMPILER_ID}" MATCHES "GNU") OR ("${CMAKE_C_COMPILER_ID}" MATCHES "Clang"))
-    add_compile_options(-ansi -pedantic -Wall -Wextra -Werror -Wstrict-prototypes -Wwrite-strings)
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -ansi -pedantic -Wall -Wextra -Werror -Wstrict-prototypes -Wwrite-strings")
 endif()
 
 #variables for pkg-config