define inline for Visual Studio

In Visual Studio, inline is available in C++ only, however __inline is available for C, see
http://msdn.microsoft.com/en-us/library/z8y1yy88.aspx

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Dr Stephen Henson <steve@openssl.org>

e_os.h

@@ -379,7 +379,16 @@ static unsigned int _strlen31(const char *str)
 #  define check_winnt() (1)
 #else
 #  define check_winnt() (GetVersion() < 0x80000000)
-#endif 
+#endif
+
+/*
+ * Visual Studio: inline is available in C++ only, however
+ * __inline is available for C, see
+ * http://msdn.microsoft.com/en-us/library/z8y1yy88.aspx
+ */
+#if defined(_MSC_VER) && !defined(__cplusplus) && !defined(inline)
+#  define inline __inline
+#endif
 
 #else /* The non-microsoft world */
 
@@ -754,4 +763,3 @@ struct servent *getservbyname(const char *name, const char *proto);
 #endif
 
 #endif
-