Add an option to autocxx_gen to create cxx.h

[nak3]

Fixes #1246

This patch adds an additional command line option to autocxx_gen to create cxx.h.

Currently cxx.h needs to be present and autocxx_gen specifies the location of cxx.h.
This patch allows users to generate cxx.h like autocxx_build.

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to README are included in PR

[nak3]

I'm adding the option name as generate-cxx-h not gen-cxx-h to distinguish the gen-* family (gen-cpp, gen-rs-include, etc) which is the mandatory options.
Please let me know if gen-cxx-h or other name should be used.

[adetaylor]

Thanks! That naming decision making sounds good to me.