libsupport: Add support_select_normalizes_timeout

It will be used on a select() test.

support/Makefile

@@ -69,6 +69,7 @@ libsupport-routines = \
   support_record_failure \
   support_run_diff \
   support_select_modifies_timeout \
+  support_select_normalizes_timeout \
   support_set_small_thread_stack_size \
   support_shared_allocate \
   support_small_stack_thread_attribute \

support/support.h

@@ -148,6 +148,10 @@ extern bool support_stat_nanoseconds (const char *path);
    no slept.  */
 extern bool support_select_modifies_timeout (void);
 
+/* Return true if select normalize the timeout input by taking in account
+   tv_usec larger than 1000000.  */
+extern bool support_select_normalizes_timeout (void);
+
 __END_DECLS
 
 #endif /* SUPPORT_H */

support/support_select_normalizes_timeout.c

@@ -0,0 +1,29 @@
+/* Return whether select normalizes the timeout.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/support.h>
+
+bool
+support_select_normalizes_timeout (void)
+{
+#ifdef __linux__
+  return true;
+#else
+  return false;
+#endif
+}