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 will add the testnet config copyNpaste to test_utils.c,
so that the test stups can set these.
Alternatively, to reduce code duplication, we can move the
testnet_config and mainnet_config from options.c to options.h.
