Make it possible to disable conf.

Now the configure script has --disable-conf command-line argument
that can be used to turn off environment variables and use of
configuration files.

libnfc/conf.c

@@ -21,6 +21,7 @@
 #  include "config.h"
 #endif // HAVE_CONFIG_H
 
+#ifdef CONF
 #include <stdio.h>
 #include <stdlib.h>
 #include <dirent.h>
@@ -189,3 +190,5 @@ conf_load(nfc_context *context)
   conf_devices_load(LIBNFC_DEVICECONFDIR, context);
 }
 
+#endif // CONF
+