[cmake][interop] add dynamic check for libstdc++fs

[aaronj0]

Based on 957ac33: Looking at the compiler version is not enough to know if we really need to link against stdc++fs:
"Whether linking with libstdc++fs is needed depends on the version of libstdc++, not on the compiler. For example, it is possible to use an older library with a newer compiler, or with another compiler that follows a different versioning scheme (for example Clang)."
594a2b8 broke some Clang builds as the need to link against stdc++fs was not a GCC issue, but to do with the libstdc++ version. This adds a dynamic check in the same fashion as in ROOT, but in CppInterOp so that standalone builds do not suffer from the same issue.

The same dynamic check if a source compiles doesn't seem to work in CppInterOp's cmake. I will investigate further, for now updating the check to use ROOT_NEED_STDCXXFS

@hahnjo can you test if this commit fixes your build?

[guitargeek]

LGTM! Thanks.

[github-actions]

Test Results

    22 files      22 suites   3d 9h 0m 33s ⏱️
 3 779 tests  3 778 ✅ 0 💤 1 ❌
75 161 runs  75 160 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 28a646c.

[hahnjo]

I tried to comment on the old commit, but that doesn't seem to show up 😒

The probable reason the check didn't work in CppInterOp integrated into ROOT is that we never reset CMAKE_REQUIRED_LIBRARIES in cmake/modules/SearchInstalledSoftware.cmake. So the variable is still there and the HAVE_NATIVE_FILESYSTEM already passes.

[hahnjo]

from #20774 (comment):

Looking at the code again, the only place using <filesystem> is https://github.com/compiler-research/CppInterOp/blob/4cc7ff5e9af52e6535d3c8f33e8fce7b6c8120b4/lib/CppInterOp/CppInterOp.cpp#L3357-L3358. Instead I think you should just call llvm::sys::fs::is_directory, which you are already using throughout the file...

[aaronj0]

Closing in favour of compiler-research/CppInterOp#797 that should no longer require linking against stdc++fs