add cpp-filesystem and cpp-experimental-filesystem dependency

[na-trium-144]

close #14249

I tested with gcc 5-10 and clang 7-10 here: https://github.com/na-trium-144/meson-fs-dep-test/actions/runs/17265187744

filesystem_dep = dependency('cpp-filesystem')

1. Checks if <filesystem> header is available.
2. Checks if test code compiles and runs without crash with <filesystem>.
   • For instance when both gcc 8 and 9 are installed, gcc 9 does not need stdc++fs, while gcc 8 without stdc++fs causes segfault. So we need to actually run the test.
   • We cannot rely on compiler versions, because clang can use libstdc++.
3. If it fails, it tries to find libstdc++fs (for gcc 8) or libc++fs (for clang 7-8) and run test again with those libraries.
4. If it is cross compilation and cannot run host binaries, it just search libstdc++fs or libc++fs and use if it's found.

filesystem_dep = dependency('cpp-experimental-filesystem')

does the same with <experimental/filesystem>, using libstdc++fs (for gcc 6-7) and libc++experimental (for clang 5-6).

Example with fallback to experimental/filesystem:

filesystem_dep = dependency('cpp-filesystem', 'cpp-experimental-filesystem')
filesystem_is_experimental = filesystem_dep.name() == 'cpp-experimental-filesystem'

executable('main', 'main.cc',
  dependencies: [filesystem_dep],
  cpp_args: filesystem_is_experimental ? ['-DEXPERIMENTAL_FS'] : []
)

#include <iostream>
#ifdef EXPERIMENTAL_FS
#include <experimental/filesystem>
namespace std_fs = std::experimental::filesystem;
#else
#include <filesystem>
namespace std_fs = std::filesystem;
#endif

int main(){
    std::cout << std_fs::exists("foo") << std::endl;
}

[jwakely]

• For instance when both gcc 8 and 9 are installed, gcc 9 does not need stdc++fs, while gcc 8 without stdc++fs causes segfault.

There should not be a segfault. If you compile with gcc-8 and don't use -lstdc++fs then you should get a linker error:

/home/jwakely/gcc/8.5.0/include/c++/8.5.0/bits/fs_path.h:185:(.text._ZNSt10filesystem7__cxx114pathC2IA4_cS1_EERKT_NS1_6formatE[_ZNSt10filesystem7__cxx114pathC5IA4_cS1_EERKT_NS1_6formatE]+0x5e): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'

Unfortunately because of the way Ubuntu packages libstdc++.so that doesn't necessarily happen. The linker finds the libstdc++.so from GCC 9 and thinks it's OK to use, and then it links to the incompatible libstdc++.so and crashes at runtime. Ubuntu probably could have done something to prevent this problem, but it's too late to fix now.

For other distros, you should get a linker error. But sadly that means for the general case, you can't rely on the linker error, and so you do need to execute the program to be sure :-(