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. See 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 #define.

  Instead, make use of ISO C concatenation of adjacent string literals, or use \n followed by a backslash-newline.