Add flatc '--cpp_std' switch (#5656)

* Add flatc '--cpp_std' switch and sandbox for C++17 code generator

- Added 'flac --cpp_std legacy' for compatibility with old compilers (VS2010);
- Added experimental switch 'flac --cpp_std c++17' for future development;
- Added C++17 sandbox test_cpp17.cpp;
- C++ code generator generates enums with explicit underlying type to avoid problems with the forward and backward schema compatibility;
- Adjusted CMakeLists.txt, CI and generate code scripts to support of introduced '--cpp_std';

* Fix --cpp_std values: c++0x, c++11, c++17

* Add 'cpp::CppStandard' enum

* Add testing engine into test_cpp17

* Rebase to upstream/master

* Set default '--cpp-std C++0x'

* Fix code generation (--cpp_std C++11) in CMakeLists.txt

- Fix dependency declaration of grpctest target

* Revert --cpp-std for the tests from explicit C++11 to flatc default value (C++0x)

include/flatbuffers/flatc.h

@@ -14,6 +14,9 @@
  * limitations under the License.
  */
 
+#ifndef FLATBUFFERS_FLATC_H_
+#define FLATBUFFERS_FLATC_H_
+
 #include <functional>
 #include <limits>
 #include <string>
@@ -22,11 +25,11 @@
 #include "flatbuffers/idl.h"
 #include "flatbuffers/util.h"
 
-#ifndef FLATC_H_
-#  define FLATC_H_
-
 namespace flatbuffers {
 
+extern void LogCompilerWarn(const std::string &warn);
+extern void LogCompilerError(const std::string &err);
+
 class FlatCompiler {
  public:
   // Output generator for the various programming languages and formats we
@@ -94,4 +97,4 @@ class FlatCompiler {
 
 }  // namespace flatbuffers
 
-#endif  // FLATC_H_
+#endif  // FLATBUFFERS_FLATC_H_