test for _sys_nerr only if sys_nerr was not found

auto/unix

diff --git a/auto/unix b/auto/unix
index 5818180..1801830 100755
--- a/auto/unix
+++ b/auto/unix
@@ -119,16 +119,18 @@
 . auto/feature
 
 
-# Cygiwn defines _sys_nerr
-ngx_feature="_sys_nerr"
-ngx_feature_name="NGX_SYS_NERR"
-ngx_feature_run=value
-ngx_feature_incs='#include <errno.h>
-                  #include <stdio.h>'
-ngx_feature_path=
-ngx_feature_libs=
-ngx_feature_test='printf("%d", _sys_nerr);'
-. auto/feature
+if [ $ngx_found = no ]; then
+    # Cygiwn defines _sys_nerr
+    ngx_feature="_sys_nerr"
+    ngx_feature_name="NGX_SYS_NERR"
+    ngx_feature_run=value
+    ngx_feature_incs='#include <errno.h>
+                      #include <stdio.h>'
+    ngx_feature_path=
+    ngx_feature_libs=
+    ngx_feature_test='printf("%d", _sys_nerr);'
+    . auto/feature
+fi
 
 
 ngx_feature="localtime_r()"