[wwwdocs] Porting to again

[Jakub Jelinek]

On Wed, Feb 18, 2015 at 01:04:30PM +0100, Marek Polacek wrote:
> --- porting_to.html	10 Feb 2015 11:12:20 -0000	1.3
> +++ porting_to.html	18 Feb 2015 12:01:50 -0000
> @@ -24,6 +24,17 @@
>  manner. Additions and suggestions for improvement are welcome.
>  </p>
>  
> +<h2>Preprocessor issues</h2>
> +
> +<p>The preprocessor started to emit line markers to properly distinguish
> +whether a macro token comes from a system header, or from a normal header
> +(see <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60723">PR60723</a>).
> +These new markers can cause intriguing problems, if the packages aren't ready
> +to handle them.  To stop the preprocessor from generating the <code>#line</code>
> +directives, use the <code>-P</code> option, documented
> +<a href="https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html#Preprocessor-Options">here</a>.
> +</p>

I think it would be nice to give here some example, like:
#include <stdlib.h>
exitfailure EXIT_FAILURE
and showing that older gcc -E used to emit
# 2 "test.c" 2
exitfailure 1
whereas GCC 5 emits:
# 2 "test.c" 2

# 2 "test.c"
exitfailure 
# 2 "test.c" 3 4
           1
and thus it can break simple tools that expect the tokens on a single line.

Otherwise, LGTM.

	Jakub