[cairo] Cleaning up cairo-features.h

[Bill Spitzak]

Glad you changed this.

cairo-demo depended plenty on #ifdef working like before, I was unable 
to get anything to compile due to this. I tried for awhile adding #undef 
to the start of the files but eventually gave up.

I have usally seen the missing data being put in the header as:

/* #undef CAIRO_HAS_XLIB_SURFACE */

This allows the user to fix the configuration by editing the file.

Carl Worth wrote:

> Yes, I botched this. I also foolishly didn't realize that the original
> version of cairo-features.h worked fine with all four of the forms
> above. I hadn't realized it's perfectly valid to test an undefined
> macro with #if, since it looks like it really wants to be an error.
> 
> 
>>Is there really a benefit from deviating from the standard 
>>of:
>>
>> #define CAIRO_HAS_XLIB_SURFACE 1
>>
>>vs.
>>
>> /* Nothing */