- Use a more conformant C++ preprocessor macro identifier for Unix systems.

The 'unix' identifier isn't defined when '-std' is passed to the compiler (tested with gcc and clang), so use '__unix__' which is well enough documented.

src/c_dispatch.cpp

@@ -1506,7 +1506,7 @@ CCMD (pullin)
 	{
 		const char *lastSlash;
 
-#ifdef unix
+#ifdef __unix__
 		lastSlash = strrchr (PullinFile, '/');
 #else
 		const char *lastSlash1, *lastSlash2;