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.

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>)