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)

appveyor.yml

@@ -10,9 +10,13 @@ environment:
     # Workaround for https://github.com/conda/conda-build/issues/636
     PYTHONIOENCODING: UTF-8
     CONDA_INSTALL_LOCN: "C:\\Miniconda35-x64"
+    CMAKE_OPTIONS: ""
+    CPP_TEST_OPTIONS: ""
 
   matrix:
     - CMAKE_VS_VERSION: "10 2010"
+      CMAKE_OPTIONS: "-DFLATBUFFERS_BUILD_LEGACY=1"
+      CPP_TEST_OPTIONS: "--std-cpp c++0x"
       MONSTER_EXTRA: "skip"
 
     - CMAKE_VS_VERSION: "12 2013"
@@ -31,7 +35,7 @@ configuration:
 
 before_build:
   - set MONSTER_EXTRA=%MONSTER_EXTRA%
-  - cmake -G"Visual Studio %CMAKE_VS_VERSION%" -DFLATBUFFERS_CODE_SANITIZE=1 .
+  - cmake . -G"Visual Studio %CMAKE_VS_VERSION%" -DFLATBUFFERS_CODE_SANITIZE=1 %CMAKE_OPTIONS%
   # This cuts down on a lot of noise generated by xamarin warnings.
   - del "C:\Program Files (x86)\MSBuild\14.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets"
 
@@ -55,7 +59,7 @@ test_script:
   - call .appveyor\check-generate-code.bat -b %CONFIGURATION%
   - "cd tests"
   - rem "Building all code"
-  - generate_code.bat -b %CONFIGURATION%
+  - generate_code.bat -b %CONFIGURATION% %CPP_TEST_OPTIONS%
   - 7z a GeneratedMyGameCode.zip MyGame\
   - rem "---------------- C++ -----------------"
   - "cd .."