Enable C++ 14 flags

[SergioRAgostinho]

This pull request enables C++14 flag for the library, effectively making a C++14 supporting compiler an official requirement. If desired, the user may request to build the library with even newer standard by passing e.g. -DCMAKE_CXX_STANDARD=17 option to CMake.

Note that the flag is not exported to PCLConfig.cmake. This means that downstream projects linking against PCL have to explicitly set the flag. This is done to support e.g. using PCL built with C++14 together with downstream projects built with C++17.

[taketwo]

We had one more item on the "switching on c++14" list:

• Add a #error in one of main headers to check if 14 is activated, this is to solve the last item mentioned here: [WIP] Moving to C++ 14 #2382 (comment)

It turned out this may be problematic on MSVC: #2382 (comment). However, I think it will still be helpful if we have such thing working for at least Ubuntu/macOS.

[taketwo]

Thanks Sérgio. I have updated the PR summary to describe what's going on in more detail.

[SunBlack]

Maybe we should force VS 2015 as minimum requirement - depends on which C++ features you want to use.

[taketwo]

Aren't we implicitly forcing this by setting CMAKE_CXX_STANDARD_REQUIRED to ON?

[SunBlack]

Never tried if this checks works, but we have implemented this just in root CMakeLists of PCL and not in PCLConfig.cmake. For installed PCL, check in pcl_config.h.in was introduced - but MSVC version check is excluded there. We could check for _MSC_VER < 1900 (see here), but with this check we don't know if it is 15.3 and below or higher (this check would be nice for e.g. #2694). But better than nothing.

[taketwo]

I see your point. I don't really have an opinion here though, I'm not a Windows user. Maybe we wait some time and see how many people show up and file issues. If there are only few, perhaps we don't need this check.

[jasjuang]

I am a bit late to the party. In order to not having downstream projects linking against PCL having to explicitly set the c++14 flag, it's a better idea to use something like

target_compile_features(${PROJECT_NAME} PUBLIC cxx_std_14)

so when the downstream target link against pcl targets the c++14 flags will be propagated along with it.

Edit: In the case where the downstream is using c++17, it won't cause any problems, the c++14 flags from pcl targets will be superseded by c++17 flags without a problem.

[SergioRAgostinho]

So the idea would be declare that in PCLConfig.cmake somewhere around here
https://github.com/PointCloudLibrary/pcl/blob/master/PCLConfig.cmake.in#L588-L627
when we declare the imported targets.

[jasjuang]

Manually setting it in PCLConfig.cmake will work too.