[test] update compile_all_aircrafts test

Also replace LOCAL_CFLAGS (which wasn't used anymore) with USER_CFLAGS (and add where missing).
Now you can e.g. test compile all aircrafts/targets in your conf and add extra flags like treating warnings as errors:

$ J=AUTO USER_CFLAGS=-Werror prove conf/examples
fix travis config

use prove instead of make run_tests so it will use all tests in tests/examples
(we can add more without changing config)
[build] don't print infos on stderr

generators should only output on stderr if there actually was an error.
Rather echo to stdout from the makefile to print the info.

This makes it possible to redirect stderr on compiling/testing to a buffer/file.
E.g. to check if there were warnings..