diff --git a/docs/libcurl/libcurl.m4 b/docs/libcurl/libcurl.m4
index 01a0575cc5..8cada0523a 100644
--- a/docs/libcurl/libcurl.m4
+++ b/docs/libcurl/libcurl.m4
@@ -157,6 +157,7 @@ x=CURLOPT_FILE;
 x=CURLOPT_ERRORBUFFER;
 x=CURLOPT_STDERR;
 x=CURLOPT_VERBOSE;
+if (x) ;
 ])],libcurl_cv_lib_curl_usable=yes,libcurl_cv_lib_curl_usable=no)
 
            CPPFLAGS=$_libcurl_save_cppflags