[gcc r11-4723] libstdc++: Fix test failure with --disable-linux-futex

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:9c1125c121423a9948fa39e71ef89ba4059a2fad

commit r11-4723-g9c1125c121423a9948fa39e71ef89ba4059a2fad
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Nov 4 15:24:47 2020 +0000

    libstdc++: Fix test failure with --disable-linux-futex
    
    As noted in PR 96817 this new test fails if the library is built without
    futexes. That's expected of course, but we might as well fail more
    obviously than a deadlock that eventually times out.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/18_support/96817.cc: Fail fail if the library is
            configured to not use futexes.

Diff:
---
 libstdc++-v3/testsuite/18_support/96817.cc | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libstdc++-v3/testsuite/18_support/96817.cc b/libstdc++-v3/testsuite/18_support/96817.cc
index f0332967831..4591a7288a5 100644
--- a/libstdc++-v3/testsuite/18_support/96817.cc
+++ b/libstdc++-v3/testsuite/18_support/96817.cc
@@ -24,6 +24,10 @@
 #include <exception>
 #include <stdlib.h>
 
+#ifndef _GLIBCXX_HAVE_LINUX_FUTEX
+# error "This test requries futex support in the library"
+#endif
+
 int init()
 {
 #if __has_include(<sys/single_threaded.h>)