o AIX 7 defines sys_nerr in errno.h, therefore <errno.h> included

in the sys_nerr test.

o When sys_nerr and _sys_nerr are missed on a particular platform
and our euristic for a maximum errno detection applied always
print the maximum errno number we reached instead of printing void.[*]

* patch from Maxim Dounin

This commit makes possible to build nginx on AIX 7.1.

auto/unix

@@ -496,7 +496,8 @@ ngx_feature_test="char buf[1]; ssize_t n; n = pwrite(1, buf, 1, 0)"
 ngx_feature="sys_nerr"
 ngx_feature_name="NGX_SYS_NERR"
 ngx_feature_run=value
-ngx_feature_incs='#include <stdio.h>'
+ngx_feature_incs='#include <errno.h>
+                  #include <stdio.h>'
 ngx_feature_path=
 ngx_feature_libs=
 ngx_feature_test='printf("%d", sys_nerr);'
@@ -538,10 +539,10 @@ if [ $ngx_found = no ]; then
                               || p == NULL
                               || strncmp(p, "Unknown error", 13) == 0)
                           {
-                              printf("%d", n);
-                              return 0;
+                              break;
                           }
-                     }'
+                      }
+                      printf("%d", n);'
     . auto/feature
 fi