--- a/configure.d/config_project_types
+++ b/configure.d/config_project_types
@@ -66,7 +66,7 @@ netsnmp_save_CFLAGS=$CFLAGS
 CFLAGS="$CFLAGS -Werror"
 
 AC_MSG_CHECKING([for the type of fd_set::fds_bits])
-for type in __fd_mask __int32_t unknown; do
+for type in fd_mask int32_t size_t; do
   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([
 #include <sys/select.h>
 #include <stddef.h>
