Allow unconfigured USB scanners on macOS.

Based on findings from Oliver Schwartz (https://alioth-lists.debian.net/pipermail/sane-devel/2012-June/029890.html)
this change allows USB scanners to be unconfigured on macOS.  This fixes
the issue of scanners being unable to be discovered because they look
to be not configured but end up getting configured later.

sanei/sanei_usb.c

@@ -1848,6 +1848,11 @@ static void libusb_scan_devices(void)
 	  continue;
 	}
 
+/* macOS won't configure several USB scanners (i.e. ScanSnap 300M) because their
+ * descriptors are vendor specific.  As a result the device will get configured
+ * later during sanei_usb_open making it safe to ignore the configuration check
+ * here on these platforms. */
+#if !defined(__APPLE__)
       if (config == 0)
 	{
 	  DBG (1,
@@ -1855,6 +1860,7 @@ static void libusb_scan_devices(void)
 	       vid, pid, busno, address);
 	  continue;
 	}
+#endif
 
       ret = libusb_get_config_descriptor (dev, 0, &config0);
       if (ret < 0)
@@ -2574,11 +2580,17 @@ sanei_usb_open (SANE_String_Const devname, SANE_Int * dn)
 	  return SANE_STATUS_INVAL;
 	}
 
+/* macOS won't configure several USB scanners (i.e. ScanSnap 300M) because their
+ * descriptors are vendor specific.  As a result the device will get configured
+ * later during sanei_usb_open making it safe to ignore the configuration check
+ * here on these platforms. */
+#if !defined(__APPLE__)
       if (config == 0)
 	{
 	  DBG (1, "sanei_usb_open: device `%s' not configured?\n", devname);
 	  return SANE_STATUS_INVAL;
 	}
+#endif
 
       result = libusb_get_device_descriptor (dev, &desc);
       if (result < 0)