Somehow, it doesn't seem to work for me.

The USBasp can be found (no target connected, thus fails):

./avrdude -C avrdude.conf -p m16 -c usbasp -P usb

avrdude: error: usbasp_transmit: Unknown error
avrdude: error: usbasp_transmit: Unknown error
avrdude: warning: cannot set sck period. please check for usbasp firmware update.
avrdude: error: usbasp_transmit: Unknown error
avrdude: error: program enable: target doesn't answer. 1
avrdude: initialization failed, rc=-1
         Double check connections and try again, or use -F to override
         this check.


avrdude done.  Thank you.

However, when passing -P usb:xxx as suggested, I do not get
a list of devices but just:

./avrdude -C avrdude.conf -p m16 -c usbasp -P usb:xxx
avrdude: Error: Invalid -P value: 'usb:xxx'
         Use -P usb:bus:device

avrdude done.  Thank you.

Nonsensical :bus:dev values are not rejected but cause a
stupid error message:

./avrdude -C avrdude.conf -p m16 -c usbasp -P usb:foo:bar
avrdude: error: could not find USB device with vid=0x16c0 pid=0x5dc vendor='www.fischl.de' product='USBasp'

avrdude done.  Thank you.

If there is no USBasp connected, it dumps core:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 802806400 (LWP 100762/avrdude)]
0x0000000801acc7a0 in strcasecmp_l () from /lib/libc.so.7
(gdb) up
#1  0x0000000000419bcd in usbasp_open (pgm=0x80286d000, port=<value optimized out>) at usbasp.c:604
604           if(strcasecmp(port, "nibobee") == 0) {
Current language:  auto; currently minimal