what is the proper syntax for selecting proxy?
-x, --proxy=[PROTOCOL://]HOST[:PORT]

assuming you have a proxy server running on 192.168.55.10 with listening ports 8333 for stratum and 8335 for getwork

I tried different combinations but always receive:
./cpuminer: no URL supplied
Try `--help' for more information.

in the easiest form should work as -x --proxy=192.168.55.10

-x --proxy=stratum://192.168.55.10:8333
-x --proxy=stratum+tcp://192.168.55.10
-x --proxy=tcp://192.168.55.10:8335
-x --proxy=//192.168.55.10

all yield same result