Remove *.o in make clean
Use clock_gettime instead of gettimeofday

Also switch to microseconds because that's what usleep expects, and
prevent issues where tick_time might be greater than TICK_LENGTH.

https://github.com/spaceboats/3001-ledboard/commit/8461b74e7f3f56bde1941424998cd288ce0d4b0e#commitcomment-8279263
https://github.com/spaceboats/3001-ledboard/commit/1a33971ef732d76657f4f4f6ca8d6a7cdcab915a#commitcomment-8279361