Set message verbosity.
The default level is PrintNothing, which means no messages are ever
printed. If you choose to increase the message verbosity level, ensure that your
application does not close the stdout/stderr file descriptors.
You are advised to set level PrintWarnings. libusb is conservative with its
message logging and most of the time, will only log messages that explain error
conditions and other oddities. This will help you debug your software.
If the LIBUSB_DEBUG environment variable was set when libusb was initialized,
this function does nothing: the message verbosity is fixed to the value in the
If libusb was compiled without any message logging, this function does nothing:
you'll never get any messages.
If libusb was compiled with verbose debug message logging, this function does
nothing: you'll always get messages from all levels.