build failed on Linux with x32 ABI

[Serguei I. Ivantsov]

>> +#if (NGX_HAVE_RTSIG)
>> +#include <sys/sysctl.h>
>> +#endif
>> +
>>
>>  #if (NGX_HAVE_POSIX_SEM)
>>  #include <semaphore.h>

>This probably needs to be moved to other event method related
>conditional includes

I did not find an example how and where conditionally include system
header. BTW, I just make the same way I found in ngx_linux_config.h :

#if (NGX_HAVE_POLL || NGX_HAVE_RTSIG)
#include <poll.h>
#endif