Skip test when on old LinuxThreads machines and using progress threads

since you can't fork() in one thread and waitpid() on the child in another,
which is what this test expects you to do.  If Linux would just implement
the stupid POSIX standard already, this wouldn't be a problem.

This commit was SVN r5482.

test/runtime/sigchld.c

@@ -43,6 +43,11 @@ int main(int argc, char *argv[])
     pid_t pid, ret;
     int status = -1;
 
+#if OMPI_ENABLE_PROGRESS_THREADS && OMPI_THREADS_HAVE_DIFFERENT_PIDS
+    printf("test not properly configured when threads have different pids\n");
+    return 77;
+#endif
+
     orte_init();
 
     pid = fork();