Require C11 (#101)

* Made CMake enforce the C11 standard during build.

rcl_lifecycle/CMakeLists.txt

@@ -46,6 +46,7 @@ call_for_each_rmw_implementation(targets GENERATE_DEFAULT)
 
 if(BUILD_TESTING)
   if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+    set(CMAKE_C_FLAGS ${CMAKE_C_FLAGS} "-std=c11")
     set(CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS} "-std=c++14")
     add_compile_options(-Wall -Wextra -Wpedantic)
   endif()