Removed redundant global.params.os field.

I hope I have untangled the checks for "native" Windows (Triple::Win32)
vs. Windows/MinGW/Cygwin (Triple::isOSWindows) correctly.

MinGW needs some default libraries as well, has to be fixed later.

dmd2/parse.c

@@ -980,11 +980,18 @@ enum LINK Parser::parseLinkage()
         }
         else if (id == Id::System)
         {
-        // LDC we configure target at runtime
-        if (global.params.os == OSWindows)
+#if IN_LLVM
+            if (global.params.targetTriple.isOSWindows())
+                link = LINKwindows;
+            else
+                link = LINKc;
+#else
+#if _WIN32
             link = LINKwindows;
-        else
+#else
             link = LINKc;
+#endif
+#endif
         }
         else
         {