This is a bit tricky: for our signing code, we don't want scriptsigs,
but to calculate the txid, we need them. For most transactions in lightning,
they're pure segwit so it doesn't matter, but funding transactions can
have P2SH-wrapped P2WPKH inputs.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
The problem with wire headers not being generated in time before stuff
depended on it turns out to be related with inclusion order of
sub-makefiles. The inclusions must preceed the use of
LIGHTNINGD_HEADERS since they append to that variable.
