Don't define ostream& operator() if already defined by Apple (Arduino-CI#171)

* Don't define ostream& operator() if already defined by Apple.

Author: James Foster

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Removed
 
 ### Fixed
+- Don't define `ostream& operator<<(nullptr_t)` if already defined by Apple
 
 ### Security
 

cpp/unittest/OstreamHelpers.h

@@ -2,4 +2,8 @@
 
 #include <ostream>
 
+#if (defined __apple_build_version__) && (__apple_build_version__ >= 12000000)
+// defined in /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/ostream:223:20
+#else
 inline std::ostream& operator << (std::ostream& out, const std::nullptr_t &np) { return out << "nullptr"; }
+#endif