Allow test_toAscii to compile under C++20

Author: jboynes

test/src/WCharacter/test_toAscii.cpp

@@ -6,8 +6,6 @@
  * INCLUDE
  **************************************************************************************/
 
-#undef _GNU_SOURCE
-
 #include <catch.hpp>
 
 #include <vector>
@@ -47,4 +45,4 @@ TEST_CASE ("toAscii(...) is called with a invalid casted ascii character", "[toA
 TEST_CASE ("toAscii(...) is called with a character larger than 1 byte", "[toAscii-04]")
 {
   REQUIRE(arduino::toAscii(0x3030) == 0x30);
-}
+}