If you can't compile libraries, I think you're limited to the time() function and its one second accuracy. (You might be able to qx() out to a command-line program that returned the time with hundredths of a second precision, but that'd probably take long enough that it would throw off your timing.

Update: Or, as mpeppler points out, you could use syscall(). It sounds like your manufacturer probably didn't include the headers or syscall.ph, but if they did this would be a good workaround.