cpp.info: Miscellaneous obsolete features

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Miscellaneous obsolete features

   Here are a few more obsolete features.
   * Attempting to paste two tokens which together do not form a valid
     preprocessing token.
     The preprocessor currently warns about this, and the resulting
     preprocessed output is undefined.  The tokens remain distinct if
     the preprocessor is being used directly by the compiler front end.
     Most of the time, when you get this warning, you will find that
     `##' is being used superstitiously, to guard against whitespace
     appearing between two tokens.  It is almost always safe to delete
     the `##'.
   * `#pragma poison'
     This is the same as `#pragma GCC poison'.  The version without the
     `GCC' prefix is deprecated.  *Note Pragmas::.
   * Multi-line string constants
     GCC currently allows a string constant to extend across multiple
     logical lines of the source file.  This extension is deprecated
     and will be removed in a future version of GCC.  Such string
     constants are already rejected in all directives apart from
     Instead, make use of ISO C concatenation of adjacent string
     literals, or use `\n' followed by a backslash-newline.