[libstdc++] Add --disable-hosted-libstdcxx, clean up conditionals

[Gabriel Dos Reis]

Bill Wendling <wendling@ncsa.uiuc.edu> writes:

| Also sprach Gabriel Dos Reis:
| } 
| } Are you also proposing we should say --enable-languages=cxx instead of
| } the straight --enable-languages=c++?  Atre you also proposinig we
| } rename the compiler from g++ to gxx?
| } 
| Please don't be ridiculous.

That consistently follows from your previous elaborations.  If it shound
ridiculous to you, then it certainly says something about your
elaborations. 

-- Gaby