diff --git a/m4/dune_common.m4 b/m4/dune_common.m4
index c6884c8acfa3ff83ec41a927344b0d0013e30e46..76b8ea36e60dd35c87ecc47c80e0a44350b0c45a 100644
--- a/m4/dune_common.m4
+++ b/m4/dune_common.m4
@@ -61,6 +61,8 @@ AC_DEFUN([DUNE_COMMON_CHECKS],
   AC_REQUIRE([AC_TYPE_SIZE_T])
   AC_REQUIRE([AC_STRUCT_TM])
 
+  dnl check for library functions
+
   AC_LANG_PUSH([C++])
   AC_CHECK_LIB([m], [pow])
   AC_CHECK_FUNCS([sqrt strchr])