Merged in antmerlino/nuttx/net-if-externC-fix (pull request #740)

Adds extern "C" guard in include/net/if.h

Approved-by: GregoryN <gnutt@nuttx.org>

include/net/if.h

@@ -247,6 +247,14 @@ struct ifconf
  * Public Function Prototypes
  *******************************************************************************************/
 
+#ifdef __cplusplus
+#define EXTERN extern "C"
+extern "C"
+{
+#else
+#define EXTERN extern
+#endif
+
 /*******************************************************************************************
  * Name: if_nametoindex
  *
@@ -282,4 +290,9 @@ unsigned int if_nametoindex(FAR const char *ifname);
 
 FAR char *if_indextoname(unsigned int ifindex, FAR char *ifname);
 
+#undef EXTERN
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __INCLUDE_NET_IF_H */