XL: Enable use of response files for includes and objects

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

This also requires updating the Makefile and Ninja generators to avoid hard-coding the @<file> syntax for response files listing include directories. Add a CMAKE_<LANG>_RESPONSE_FILE_FLAG variable that compiler information modules can set to use a custom flag for response files.

The Makefile generator update was discussed on the dev list:

• Jan 2018: CMAKE :: mingw make files with response file for includes