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

[Bill Wendling]

Also sprach Gabriel Dos Reis:
} 
} "-" is translated to "_".  "+" could just be translated to  "x".  And
} we would get a valid shell variable name.
} 
Phil answered your previous question about my "side-tracking" things.
For this, I suggest you write a patch for the autoconf people so that it
does what you suggest. As it is, it doesn't presently do that.

-- 
|| Bill Wendling            "Real Programmers have a Snoopy Calendar
|| wendling@ncsa.uiuc.edu    of '69 hanging on their wall"
|| Coding Simian                       -- Toon Moene