And turn "" includes into full-path (which makes it easier to put
config.h first, and finds some cases check-includes.sh missed
previously).
config.h sets _GNU_SOURCE which really needs to be done before any
'#includes': we mainly got away with it with glibc, but other platforms
like Alpine may have stricter requirements.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This builds on the enctlv vectors, but actually goes all the way
to creating a modern onionmessage.
Thanks to Thomas H for corrections!
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>