Write only file basename to log messages

[sethrj]

Full paths from the celeritas logger are nice when debugging but annoying in most cases due to obscuring the rest of the message. In accel/detail/LoggerImpl.cc we have a function to remove these paths at runtime, but it would be more sensible to have an option like this at compile time to (1) avoid having to use -ffile-prefix-map to the compiler to improve ccache behavior and (2) avoid having often arbitrary source paths compiled into the executable.