[PATCH] Configure: don't test alternatives if preferred feature is found

[Piotr Sikora]

Hey Ruslan,

> So far I have mixed feelings about this patch, but if it gets
> accepted then the compile-time condition for ngx_read_ahead()
> in src/os/unix/ngx_files.c can be simplified to only test
> NGX_HAVE_POSIX_FADVISE.  Also, there is the currently unused
> macro NGX_HAVE_READ_AHEAD.

Good idea, thanks!

Best regards,
Piotr Sikora