Fixes to improve bare metal compilation

[biojppm]

re #63

[codecov]

Codecov Report

Merging #64 (aead923) into master (fe1ad5f) will increase coverage by 0.13%.
The diff coverage is 15.38%.

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   93.02%   93.16%   +0.13%     
==========================================
  Files          50       49       -1     
  Lines       10107    10089      -18     
==========================================
- Hits         9402     9399       -3     
+ Misses        705      690      -15     

Impacted Files	Coverage Δ
src/c4/ext/fast_float_all.h	28.12% <0.00%> (+0.38%)	⬆️
src/c4/substr.hpp	98.83% <ø> (-0.01%)	⬇️
src/c4/charconv.hpp	97.58% <100.00%> (+<0.01%)	⬆️

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