some pretty safe & reasonable defaults to speed up SCons:
- look at timestamps to figure out whether a file has changed
- but only if the timestamp is older than 10 secs (which should avoid any problems with 1s precision of mtime, as well as clock drifts etc)
- implicit cache is something to do with parsing C files for dependencies
see https://github.com/SCons/scons/wiki/GoFastButton
