Add workaround for Objective-C compatibility

Protect against Objective C's `#define Nil nullptr` by temporarily
suppressing the Nil define and providing an alias for Nil ("Nill") to
support calling from Objective C contexts.

See #1

src/uuidxx.cpp

@@ -2,6 +2,9 @@
 #define _CRT_SECURE_NO_WARNINGS
 #endif
 
+// ObjC compatibility issue
+#undef Nil
+
 #include "uuidxx.h"
 #include <random>
 #include <stdio.h>

src/uuidxx.h

@@ -1,5 +1,9 @@
 #pragma once
 
+// ObjC compatibility
+#pragma push_macro("Nil")
+#undef Nil
+
 #include <stdexcept>
 #include <cstdint>
 #include <string>
@@ -9,6 +13,7 @@ namespace uuidxx
 	enum class Variant
 	{
 		Nil,
+		Nill, // for ObjC compatibility
 		Version1,
 		Version2,
 		Version3,
@@ -61,6 +66,7 @@ namespace uuidxx
 			switch (v)
 			{
 			case Variant::Nil:
+			case Variant::Nill:
 				return uuid(nullptr); // Special-cased
 			case Variant::Version1:
 			case Variant::Version2:
@@ -79,3 +85,5 @@ namespace uuidxx
 
 	static_assert(sizeof(uuid) == 2 * sizeof(int64_t), "Check uuid type declaration/padding!");
 }
+
+#pragma pop_macro("Nil")