Fix: std::filesystem include issues

[dragonmux]

As part of getting CI for bmpflash up and working, we encountered a series of issues surrounding and getting the checks for it being enabled right. This PR seeks to improve this situation by, among other things, including the header to ensure the proper feature macros are available in <substrate/include/defs> and improve the logic for defining the guard macro.

The result should be a much more robust path to getting SUBSTRATE_ALLOW_STD_FILESYSTEM_PATH defined in appropriate circumstances.

[codecov]

Codecov Report

Merging #76 (944b65b) into main (6483ea5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #76   +/-   ##
=======================================
  Coverage   90.38%   90.38%           
=======================================
  Files          47       47           
  Lines        3277     3277           
  Branches      644      644           
=======================================
  Hits         2962     2962           
  Misses        247      247           
  Partials       68       68           

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