Set the C++ version to 17. (#761)

Even though this is a C package, we still use C++
for the tests.  So we should set the version to C++17.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

rosidl_runtime_c/CMakeLists.txt

@@ -7,6 +7,12 @@ if(NOT CMAKE_C_STANDARD)
   set(CMAKE_C_STANDARD 11)
 endif()
 
+# Default to C++17
+if(NOT CMAKE_CXX_STANDARD)
+  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+endif()
+
 if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_C_COMPILER_ID MATCHES "Clang")
   add_compile_options(-Wall -Wextra -Wpedantic)
 endif()