Please implement GLR + C++ + variant or at least warn user that it is unavailable

[Askar Safin]

Hi. Please add variant support to GLR C++ parser. Reason is following:
* Having C++ parser is cool. It allows us to have non-trivial C++ objects as 
semantic values
* But then (if we use non-trivial objects as semantic values) we need variant
* And (as docs say) nearly any modern programming language needs GLR parser and 
not LARL1
If you don't plan to add this support at near future, then, please, put big fat 
warning to docs that GLR + C++ + variant is not supported at the beginning of 
C++ section. If I knew this earlier I probably would not convert my C parser to 
C++ parser. Please, CC me if you want to answer

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Askar Safin
http://vk.com/safinaskar
Kazan, Russia