CMake support for shared library and Visual Studio 2019

[paulhoux]

This PR adds support for building Cinder as a shared library using the CMake system. It also adds support for Visual Studio 2019.

See also the discussion around PR #2117 . My PR also removes the forced /MT setting and can therefore be used instead of #2117.

The CMake files for Windows were missing the following files in the DLL build:

• videoInput/videoInput.cpp

The following files were specifically excluded from the DLL build:

• All of AntTweakBar's files
• cinder/ImageSourcePng.cpp
• cinder/app/AppScreenSaver.cpp
• cinder/app/msw/AppInplMswScreenSaver.cpp

The following libraries were added as dependencies in the DLL build:
kernel32.lib,user32.lib,gdi32.lib,winspool.lib,comdlg32.lib,advapi32.lib,shell32.lib,ole32.lib,oleaut32.lib,uuid.lib,odbc32.lib,odbccp32.lib

A build option was added: CINDER_ENABLE_CXX17 (defaults to OFF). Setting this to ON will attempt to build Cinder using the C++17 language. For Visual Studio 2019, this means it will use the proper <filesystem> header, instead of <experimental/filesystem>.

Important! This PR has only been tested on Windows. Before accepting it, please make sure it does not break building on the other platforms!

[MikeGitb]

Why is CINDER_ENABLE_CXX17 (and the whole associated machinery) necessary, when cmake already has CMAKE_CXX_STANDARD?

[paulhoux]

Visual Studio 2019 defaults to C++14, so I wanted to allow people to opt for C++17 instead. Most of the "machinery" was already in the make files, but I guess using CMAKE_CXX_STANDARD for that is indeed cleaner.

[MikeGitb]

Personally I think you could plain remove everything from line 89 to 134. It is essentially just duplicating existing cmake functionality.

But I haven't checked myself if those heads any bad sideeffect.

[paulhoux]

Doesn't that break compatibility with Visual Studio 2015? I've looked at the current version of the cmake file and those lines were added in #1594 . According to CMake's docs, prior to Visual Studio 2015 update 3 the CXX_STANDARD flag doesn't do anything.

[MikeGitb]

Yes, because VS2015 doesn't have different language modes in the first place.

[MikeGitb]

Btw. I believe that _HAS_CXX17 is a STL-internal macro that should not be used in user code.

https://stackoverflow.com/questions/52379233/is-has-cxx17-marco-usable-in-custom-project-headers-to-enable-c17-language

I haven't checked, when this was introduced, but for conditionally enabling std::filesystem, the standard featur test macro __cpp_lib_filesystem might be a better choice. Alternatively there exists _MSVC_LANG mentioned in the linked SO thread.

[paulhoux]

Thanks, Mike. Seems I still have a lot to learn.

[MikeGitb]

Mostly minor stuff I'd say. I also just realized that the scrip claims compatibility with cmake 2.9 (I wonder if anyone is actually verifying this - 2.9 was realeased around 2010 ) which I believe didn't have CMAKE_CXX_STANDARD.

Anyway, I think a cmake overhaul has to wait until after 9.3 got released, but I'd very much appreciate if this PR would find its way into 9.3.

[EDIT: that was supposed to mean 9.2]

[godefv]

I have used cinder on linux without any issue.
Then, on Windows, it is broken because it is incompatible with C++17 and C++20.

Thanks to this PR, I can compile cinder with C++17, but still not C++20, because std::result_of has been removed in C++20. We now need to use std::invoke_result, which is not there before C++17.

It is a pain to keep supporting every C++ standard. So, I think that, since C++14 is quite old now, there could be a last release supporting it, before dropping C++14 support for future releases, in favor of C++17 and C++20. It would make cinder only fresher and more attractive.

[MikeGitb]

Could you open an issue about the c++20 breakage - ideally with the error message and or line, where result_of is used?

[godefv]

Let me provide more detail.

With GCC 10.2 on linux, I can compile with C++20. It seems to have kept backward compatibility, despite the C++20 standard not requiring it.
With Clang or VS2019 on Windows 10, compilation fails. There a couple of other issues in addition to result_of.

You can check here (godefv@f33b943) what I have done to successfully compile with C++20. This commit is on top of this PR.

EDIT: I have put this comment in a new issue as you have asked.