Build issues from filesystem header include in C++ API

[psakievich]

I'm unable to compile the dev branch due to the include of filesystem that was added in #670. I'm using GCC 7.2 and Intel 19. Is the include of filesystem necessary? This header is not supported in GCC 7.

If necessary I can move my build systems up to a GCC 8 if we decide we don't need to support GCC 7 for Exawind/HFM.
@sayerhs or @jrood-nrel would you care to weigh in on this?

I guess some macro magic could be employed to switch the include to experimental/filesystem too.

[rafmudaf]

Do you need to compile the C++ based driver or just the C++ API? If just the API, you shouldn't need to deal with FAST_Prog.cpp and its included filesystem. That make-target for the API library is openfastcpplib whereas openfastcpp compiles both the API and the driver code.

[sayerhs]

@rafmudaf Sorry I commented on #670. I'll reproduce that here.

• Is C++17 the minimum standard required for OpenFAST? filesystem is only supported in C++17 so if we configure it with -DCMAKE_CXX_STANDARD=14 the build will fail.
• openfastcpp executable is part of the install targets

  openfast/glue-codes/openfast-cpp/CMakeLists.txt

  Lines 85 to 86 in 718d46f

  	install(TARGETS openfastcpp
  	RUNTIME DESTINATION bin)

  It is not sufficient that we can build openfastcpplib we need to be able to execute make install. So it seems like a breaking change.

[sayerhs]

Also to @psakievich 's point, why is this header included? I don't see any invocations of std::filesystem anywhere.

[rafmudaf]

These are all good points.

As I understand it (from @gantech), the C++ driver is only used for testing purposes and as a tool to develop the C++ API. So, if anything needs to change there, I don't see a problem with it. I'll take out this unused import of filesystem.

It's also worth considering removing it from the install target.

[rafmudaf]

@psakievich I've updated this in dev. Please let me know if you're able to compile now.

[psakievich]

@rafmudaf thanks. I just tested and it builds fine now. This resolves the issue for me.