diff --git a/src/modules/files/configure.in b/src/modules/files/configure.in
index a5c3c94e1b3619f27b91b7bc88bface08e4e5242..796f18529502b18f28669a76ef3b48f6cebae7af 100644
--- a/src/modules/files/configure.in
+++ b/src/modules/files/configure.in
@@ -16,7 +16,7 @@ if test x$ac_cv_func_readdir_r = xyes ; then
   AC_MSG_CHECKING(if readdir_r takes 2(Solaris & HPUX) or 3(Posix) arguments)
   AC_CACHE_VAL(pike_cv_func_posix_readdir_r, [
     AC_TRY_RUN([
-// For _REENTRANT
+/* For _REENTRANT */
 #include "machine.h"
 #ifndef POSIX_SOURCE
 #define POSIX_SOURCE
@@ -60,7 +60,7 @@ else
   AC_MSG_CHECKING(if readdir_r returns an int(HPUX) or a pointer(Solaris))
   AC_CACHE_VAL(pike_cv_func_solaris_style_readdir_r, [
     AC_TRY_RUN([
-// For _REENTRANT
+/* For _REENTRANT */
 #include "machine.h"
 #ifndef POSIX_SOURCE
 #define POSIX_SOURCE