src: add check for already defined macro NOMINMAX

In order to avoid Visual C++ warning C4005 about macro redefinition
when node.h is included in another project.

PR-URL: https://github.com/iojs/io.js/pull/986
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/node.h

@@ -25,7 +25,9 @@
 # define _WIN32_WINNT   0x0501
 #endif
 
-#define NOMINMAX
+#ifndef NOMINMAX
+# define NOMINMAX
+#endif
 
 #endif