Support clang and std::filesystem

[nkoenig]

🎉 New feature

Summary

Support clang when checking for filesystem support.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #390 (3d5471a) into ign-transport8 (cb1c95b) will increase coverage by 0.00%.
The diff coverage is 88.37%.

❗ Current head 3d5471a differs from pull request most recent head ce80df6. Consider uploading reports for the commit ce80df6 to get more accurate results

@@               Coverage Diff               @@
##           ign-transport8     #390   +/-   ##
===============================================
  Coverage           83.61%   83.61%           
===============================================
  Files                  51       51           
  Lines                5035     5036    +1     
===============================================
+ Hits                 4210     4211    +1     
  Misses                825      825           

Impacted Files	Coverage Δ
include/gz/transport/Packet.hh	0.00% <0.00%> (ø)
...og/include/gz/transport/log/detail/QueryOptions.hh	100.00% <ø> (ø)
log/src/Batch.cc	89.28% <ø> (ø)
log/src/Descriptor.cc	89.65% <ø> (ø)
log/src/Descriptor.hh	100.00% <ø> (ø)
log/src/Log.cc	78.72% <ø> (ø)
log/src/Message.cc	100.00% <ø> (ø)
log/src/MsgIter.cc	80.43% <ø> (ø)
log/src/QualifiedTime.cc	97.81% <ø> (ø)
log/src/QueryOptions.cc	100.00% <ø> (ø)
... and 40 more

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