lib: Use GLib and libusb directly for debug output

Use GLib internally to output debug information, and tell about
libusb's LIBUSB_DEBUG envvar for libusb debug.

fp_set_debug() is now a no-op.

https://bugs.freedesktop.org/show_bug.cgi?id=106552

libfprint/drivers/elan.c

@@ -20,10 +20,11 @@
 
 #define FP_COMPONENT "elan"
 
+#include "fp_internal.h"
+
 #include <errno.h>
 #include <libusb.h>
 #include <assembling.h>
-#include <fp_internal.h>
 #include <fprint.h>
 
 #include "elan.h"