XL: Enable use of response files for includes and objects

The IBM XL compiler supports response files via `-qoptfile=<file>`
instead of `@<file>`.

Modules/Compiler/XL.cmake

@@ -20,6 +20,8 @@ macro(__compiler_xl lang)
   # Feature flags.
   set(CMAKE_${lang}_VERBOSE_FLAG "-V")
   set(CMAKE_${lang}_COMPILE_OPTIONS_PIC "-qpic")
+  set(CMAKE_${lang}_RESPONSE_FILE_FLAG "-qoptfile=")
+  set(CMAKE_${lang}_RESPONSE_FILE_LINK_FLAG "-qoptfile=")
 
   string(APPEND CMAKE_${lang}_FLAGS_DEBUG_INIT " -g")
   string(APPEND CMAKE_${lang}_FLAGS_RELEASE_INIT " -O")