Libmv: Add threading primitives

Allows to use mutex, scoped_lock, and conditional_variable from within
the libmv namespace.

Implementation is coming from C++11. Other configurations are easy to
implement, but currently C++11 is the way to go.

intern/libmv/libmv/threading/threading.h

@@ -0,0 +1,43 @@
+// Copyright (c) 2020 libmv authors.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to
+// deal in the Software without restriction, including without limitation the
+// rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+// sell copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+// IN THE SOFTWARE.
+
+#ifndef LIBMV_THREADING_THREADING_H_
+#define LIBMV_THREADING_THREADING_H_
+
+#include "libmv/build/build_config.h"
+
+#if COMPILER_SUPPORTS_CXX11
+#  include <condition_variable>
+#  include <mutex>
+#endif
+
+namespace libmv {
+
+#if COMPILER_SUPPORTS_CXX11
+using mutex =  std::mutex;
+using scoped_lock = std::unique_lock<std::mutex>;
+using condition_variable = std::condition_variable;
+#else
+#  error Please add support for threading in threading/threading.h
+#endif
+
+}  // namespace libmv
+
+#endif  // LIBMV_THREADING_THREADING_H_