isAscii: avoid dependency on _GNU_SOURCE

[facchinm]

@DhruvaG2000
@beriberikix
@carlosperate
@matthijskooijman

[codecov-commenter]

Codecov Report

Base: 96.11% // Head: 96.20% // Increases project coverage by +0.08% 🎉

Coverage data is based on head (981a9f6) compared to base (c8ee90f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   96.11%   96.20%   +0.08%     
==========================================
  Files          13       13              
  Lines         953      949       -4     
==========================================
- Hits          916      913       -3     
+ Misses         37       36       -1     

Impacted Files	Coverage Δ
api/WCharacter.h	100.00% <100.00%> (ø)
api/Print.cpp	95.20% <0.00%> (-0.03%)	⬇️
api/Stream.cpp	91.42% <0.00%> (+0.39%)	⬆️

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[carlosperate]

Thanks for the PR @facchinm!

I haven't test it yet, but doesn't it also need an custom implementation of toascii(), as used in toAscii()?

ArduinoCore-API/api/WCharacter.h

Lines 144 to 150 in e26862e

	// Converts c to a 7-bit unsigned char value that fits into the
	// ASCII character set, by clearing the high-order bits.
	inline int toAscii(int c)
	{
	return toascii (c);
	}

[facchinm]

@carlosperate thanks for pointing that out, I added the patch and test for that function too