patch: configure and libusb-config bug

[Paul B. <peb...@sa...>]

In most cases, configure uses the variable 'LIBUSB_CONFIG' to determine 
the location of 'libusb-config' . However, in two cases, configure uses 
'libusb-config' directly rather than the variable 'LIBUSB_CONFIG'. This 
means that the variable 'LIBUSB_CONFIG' cannot be used to set the path 
to 'libusb-config'. Attached is a patch for the 0.8.0pre1 configure.in 
file that fixes this problem.