Add a script to create a ccan source tree for inclusion into projects'
distributed code.

create-ccan-tree takes a list of ccan modules and copies them, plus any
dependent modules, into a target directory. Enough build infrastructure
is added to build libccan.a from the ccan modules.

Typical usage for my project that uses talloc and read-write-all:

  lib/ccan.git/tools/create-ccan-tree --exclude-tests \
		lib/ccan/ talloc read-write-all

- this uses a git submodule in $PWDlib/ccan.git to create a source tree
in $PWD/lib/ccan, containing code for talloc and read-write-all.
Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>