Allow disabling use of <filesystem> on C++17

[SadieCat]

Description

Please provide a detailed description of what was done in this PR.

Changes include

• [ ] Bugfix (non-breaking change that solves an issue)
• [x] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (change that is not backwards-compatible and/or changes current functionality)

Checklist

• [x] I have assigned this PR to myself
• [x] I have added at least 1 reviewer
• [x] I have tested this code
• [x] I have added sufficient documentation in the code

Additional comments

<filesystem> is quite a big header. On my system echo '#include <filesystem>' | c++ -E -x c++ - | wc -l shows that it is 50K lines of code. Including such a large header unnecessarily in projects with a lot of translation units can affect build times quite significantly. Therefore, I added an escape hatch that allows such projects to opt-out of std::filesystem support if they don't need it.

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (718b571) compared to base (eb3acad). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #66 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 59 55 -4 Branches 3 3 ========================================= - Hits 59 55 -4 ``` | [Impacted Files](https://codecov.io/gh/martin-olivier/dylib/pull/66?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martin+Olivier) | Coverage Δ | | |---|---|---| | [include/dylib.hpp](https://codecov.io/gh/martin-olivier/dylib/pull/66/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martin+Olivier#diff-aW5jbHVkZS9keWxpYi5ocHA=) | `100.00% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martin+Olivier). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Martin+Olivier)

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