Fix build on CentOS 7 / x86_64.

The issue is that the default compiler on CentOS 7 (and presumably
RHEL 7) is an old GCC that's too old to support atomic types.
In this case we attempt to approximate an atomic 64-bit type with
a normal 64-bit integer, and warn at runtime if the system doesn't
support lock-free operations with this data type.

This has been compile-tested and lightly run-tested on CentOS 7
x86_64. It might not work well with ancient non-GCC compilers or
32-bit hosts.

src/iperf.h

@@ -77,12 +77,18 @@
 #include <pthread.h>
 #endif // HAVE_PTHREAD
 
+/*
+ * Atomic types highly desired, but if not, we approximate what we need
+ * with normal integers and warn.
+ */
 #ifdef HAVE_STDATOMIC_H
 #include <stdatomic.h>
+#else
+#warning "No <stdatomic.h> available."
+typedef uint64_t atomic_uint_fast64_t;
 #endif // HAVE_STDATOMIC_H
 
 #if !defined(__IPERF_API_H)
-//typedef uint64_t iperf_size_t;
 typedef uint_fast64_t iperf_size_t;
 typedef atomic_uint_fast64_t atomic_iperf_size_t;
 #endif // __IPERF_API_H

src/iperf_api.h

@@ -38,8 +38,15 @@
 extern "C" { /* open extern "C" */
 #endif
 
+/*
+ * Atomic types highly desired, but if not, we approximate what we need
+ * with normal integers and warn.
+ */
 #ifdef HAVE_STDATOMIC_H
 #include <stdatomic.h>
+#else
+#warning "No <stdatomic.h> available"
+typedef u_int64_t atomic_uint_fast64_t;
 #endif // HAVE_STDATOMIC_H
 
 struct iperf_test;

src/main.c

@@ -1,5 +1,5 @@
 /*
- * iperf, Copyright (c) 2014-2022, The Regents of the University of
+ * iperf, Copyright (c) 2014-2023, The Regents of the University of
  * California, through Lawrence Berkeley National Laboratory (subject
  * to receipt of any required approvals from the U.S. Dept. of
  * Energy).  All rights reserved.
@@ -59,6 +59,24 @@ main(int argc, char **argv)
 {
     struct iperf_test *test;
 
+    /*
+     * Atomics check. We prefer to have atomic types (which is
+     * basically on any compiler supporting C11 or better). If we
+     * don't have them, we try to approximate the type we need with a
+     * regular integer, but complain if they're not lock-free. We only
+     * know how to check this on GCC. GCC on CentOS 7 / RHEL 7 is the
+     * targeted use case for these check.
+     */
+#ifndef HAVE_STDATOMIC_H
+#ifdef __GNUC__
+    if (! __atomic_always_lock_free (sizeof (u_int64_t), 0)) {
+#endif // __GNUC__
+        fprintf(stderr, "Warning: Cannot guarantee lock-free operation with 64-bit data types\n");
+#ifdef __GNUC__
+    }
+#endif // __GNUC__
+#endif // HAVE_STDATOMIC_H
+
     // XXX: Setting the process affinity requires root on most systems.
     //      Is this a feature we really need?
 #ifdef TEST_PROC_AFFINITY