Echo compiler commands (useful for debugging)

We may want to wrap that into a conditional with some Make variable
VERBOSE in case this doesn't make everyone happy.
Delete all Linux build files from the working tree

Build files have not their place in the source code. It confuses the
developer as git keeps track of them, IMO. It also makes the source
code larger and the time to send/receive the commits over the network
longer.
Remove useless extra -I in the compiling option

Nothing harmless except that extra -I would generate a warning.