diff --git a/auto/lib/google-perftools/conf b/auto/lib/google-perftools/conf
index 5d5ddaef6..7f1a91139 100644
--- a/auto/lib/google-perftools/conf
+++ b/auto/lib/google-perftools/conf
@@ -9,7 +9,8 @@
     ngx_feature_incs=
     ngx_feature_path=
     ngx_feature_libs="-lprofiler"
-    ngx_feature_test="ProfilerStop()"
+    ngx_feature_test="void ProfilerStop(void);
+                      ProfilerStop()"
     . auto/feature
 
 
diff --git a/auto/lib/libgd/conf b/auto/lib/libgd/conf
index 87761f14d..678639767 100644
--- a/auto/lib/libgd/conf
+++ b/auto/lib/libgd/conf
@@ -9,7 +9,8 @@
     ngx_feature_incs="#include <gd.h>"
     ngx_feature_path=
     ngx_feature_libs="-lgd"
-    ngx_feature_test="gdImagePtr img = gdImageCreateFromGifPtr(1, NULL);"
+    ngx_feature_test="gdImagePtr img = gdImageCreateFromGifPtr(1, NULL);
+                      (void) img"
     . auto/feature
 
 
@@ -76,7 +77,8 @@ if [ $ngx_found = yes ]; then
 
     ngx_feature="GD WebP support"
     ngx_feature_name="NGX_HAVE_GD_WEBP"
-    ngx_feature_test="gdImagePtr img = gdImageCreateFromWebpPtr(1, NULL);"
+    ngx_feature_test="gdImagePtr img = gdImageCreateFromWebpPtr(1, NULL);
+                      (void) img"
     . auto/feature
 
 else
diff --git a/auto/lib/libxslt/conf b/auto/lib/libxslt/conf
index 3a0f37be5..3063ac7ce 100644
--- a/auto/lib/libxslt/conf
+++ b/auto/lib/libxslt/conf
@@ -16,8 +16,8 @@
     ngx_feature_libs="-lxml2 -lxslt"
     ngx_feature_test="xmlParserCtxtPtr    ctxt = NULL;
                       xsltStylesheetPtr   sheet = NULL;
-                      xmlDocPtr           doc;
-                      doc = xmlParseChunk(ctxt, NULL, 0, 0);
+                      xmlDocPtr           doc = NULL;
+                      xmlParseChunk(ctxt, NULL, 0, 0);
                       xsltApplyStylesheet(sheet, doc, NULL);"
     . auto/feature
 
diff --git a/auto/os/linux b/auto/os/linux
index 2c8a9bb8e..5e280eca7 100644
--- a/auto/os/linux
+++ b/auto/os/linux
@@ -185,6 +185,8 @@ ngx_feature_test="struct __user_cap_data_struct    data;
                   data.effective = CAP_TO_MASK(CAP_NET_RAW);
                   data.permitted = 0;
 
+                  (void) header;
+                  (void) data;
                   (void) SYS_capset"
 . auto/feature