diff --git a/ext/cpdf/config.m4 b/ext/cpdf/config.m4
index 7ec7a13c5a6..4f9a8e5cbc6 100644
--- a/ext/cpdf/config.m4
+++ b/ext/cpdf/config.m4
@@ -53,7 +53,7 @@ AC_ARG_WITH(cpdflib,
   PHP_WITH_SHARED
   if test "$withval" != "no"; then
     cpdf_withval=$withval
-    PHP_NEW_EXTENSION(cpdf, cpdf.c, $ext_shared)
+    PHP_NEW_EXTENSION(cpdf, cpdf.c, $ext_shared,, \\$(GDLIB_CFLAGS))
     PHP_SUBST(CPDF_SHARED_LIBADD)
     CPDF_JPEG_TEST
     CPDF_TIFF_TEST