add libgd include path

auto/lib/libgd/conf

diff --git a/auto/lib/libgd/conf b/auto/lib/libgd/conf
index 3a52770..6378723 100644
--- a/auto/lib/libgd/conf
+++ b/auto/lib/libgd/conf
@@ -65,6 +65,7 @@
 
 if [ $ngx_found = yes ]; then
 
+    CORE_INCS="$CORE_INCS $ngx_feature_path"
     CORE_LIBS="$CORE_LIBS $ngx_feature_libs"
 
 else