[libstdc++] Different version tag for libsupc++, document , config option

[Gabriel Dos Reis]

Mark Mitchell <mark@codesourcery.com> writes:

| (There seem to be two spellings for C++ that are legal identifiers:
| CPP, and CXX.  The former is C Plus Plus, the second is plus signs
| turned on an angle.)

I always tought that CXX stood for "C excess".

Sorry, I couldn't resist :-)

-- Gaby
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