Build: require C11/C++11 for all operating systems in master.

This is in preparation of upgrading our library dependencies, some of which
need C++11. We already use C++11 in blender2.8 and for Windows and macOS, so
this just affects Linux.

On many distributions this will not require any changes, on some
install_deps.sh will need to be run again to rebuild libraries.

Differential Revision: https://developer.blender.org/D3568

intern/cycles/util/util_thread.cpp

@@ -26,11 +26,7 @@ thread::thread(function<void(void)> run_cb, int group)
     joined_(false),
 	group_(group)
 {
-#if (__cplusplus > 199711L) || (defined(_MSC_VER) && _MSC_VER >= 1800)
 	thread_ = std::thread(&thread::run, this);
-#else
-	pthread_create(&pthread_id_, NULL, run, (void*)this);
-#endif
 }
 
 thread::~thread()
@@ -64,7 +60,6 @@ void *thread::run(void *arg)
 bool thread::join()
 {
 	joined_ = true;
-#if (__cplusplus > 199711L) || (defined(_MSC_VER) && _MSC_VER >= 1800)
 	try {
 		thread_.join();
 		return true;
@@ -72,9 +67,6 @@ bool thread::join()
 	catch (const std::system_error&) {
 		return false;
 	}
-#else
-	return pthread_join(pthread_id_, NULL) == 0;
-#endif
 }
 
 CCL_NAMESPACE_END