Update minimum required C++ version for Ceres to C++14

- Removes all workarounds for pre-C++14 versions
- Removes '11' qualifier from C++ threading option and associated
  defines.
- Fix missing inclusion of 'Multithreading' in reported Ceres components
  when C++ threading model is enabled.
- Update Sphinx documentation to specify C++14 as minimum requirement.

Change-Id: I706c8b367b3221e3c4d1a0aaf669a8f9c911e438

internal/ceres/thread_pool.cc

@@ -31,7 +31,7 @@
 // This include must come before any #ifndef check on Ceres compile options.
 #include "ceres/internal/port.h"
 
-#ifdef CERES_USE_CXX11_THREADS
+#ifdef CERES_USE_CXX_THREADS
 
 #include "ceres/thread_pool.h"
 
@@ -113,4 +113,4 @@ void ThreadPool::Stop() {
 }  // namespace internal
 }  // namespace ceres
 
-#endif // CERES_USE_CXX11_THREADS
+#endif // CERES_USE_CXX_THREADS