Make Sys_SetEnv behave alike on Windows and Linux

code/sys/sys_win32.c

@@ -772,7 +772,10 @@ set/unset environment variables (empty value removes it)
 */
 void Sys_SetEnv(const char *name, const char *value)
 {
-	_putenv(va("%s=%s", name, value));
+	if(value)
+		_putenv(va("%s=%s", name, value));
+	else
+		_putenv(va("%s=", name));
 }
 
 /*