build: fallback to __sync_synchronize()

If we're running on a GCC older than 4.9, then we won't have the
stdatomic.h available. We can just use a full barrier instead using
__sync_synchronize() to get the same effect, albeit slower.

meson.build

@@ -181,7 +181,7 @@ foreach link_arg: test_link_args
 endforeach
 add_project_link_arguments(global_link_args, language: 'c')
 
-if not cc.links('''
+if cc.links('''
 #include <stdatomic.h>
 int main(void) {
   atomic_thread_fence(memory_order_acquire);
@@ -189,7 +189,7 @@ int main(void) {
   return 0;
 }
 ''')
-  error('Sysprof requires a C compiler with stdatomic support such as GCC 4.9 or newer')
+  config_h.set10('HAVE_STDATOMIC_H', 1)
 endif
 
 subdir('src')

src/libsysprof-capture/mapped-ring-buffer.c

@@ -22,7 +22,6 @@
 
 #include "config.h"
 
-#include <stdatomic.h>
 #include <sys/mman.h>
 #include <sys/types.h>
 #include <unistd.h>

src/libsysprof/sysprof-perf-counter.c

@@ -42,7 +42,9 @@
 #include <errno.h>
 #include <gio/gio.h>
 #include <gio/gunixfdlist.h>
-#include <stdatomic.h>
+#ifdef HAVE_STDATOMIC_H
+# include <stdatomic.h>
+#endif
 #include <string.h>
 #include <sys/ioctl.h>
 #include <sys/mman.h>
@@ -219,7 +221,11 @@ sysprof_perf_counter_flush (SysprofPerfCounter     *self,
   tail = info->tail;
   head = info->map->data_head;
 
+#ifdef HAVE_STDATOMIC_H
   atomic_thread_fence (memory_order_acquire);
+#elif G_GNUC_CHECK_VERSION(3, 0)
+  __sync_synchronize ();
+#endif
 
   if (head < tail)
     tail = head;
@@ -285,7 +291,12 @@ sysprof_perf_counter_flush (SysprofPerfCounter     *self,
 
   info->tail = tail;
 
+#ifdef HAVE_STDATOMIC_H
   atomic_thread_fence (memory_order_seq_cst);
+#elif G_GNUC_CHECK_VERSION(3, 0)
+  __sync_synchronize ();
+#endif
+
   info->map->data_tail = tail;
 }