[ccan] Module dependency graph

[Joseph Adams]

I wrote a little Perl script for producing a graph of CCAN module
dependencies with Graphviz.  Except for a little tweak needed to get
the tools/ccan_depends --compile flag to work, the attached patch
simply drops the script in the tools directory.  I didn't add any
makefile magic.

Usage (requires Graphviz; will take several seconds):

tools/module-dep-graph.pl | dot -Tpng > out.png && xdg-open out.png

Result:

http://tinyurl.com/6jk4cbo

Better yet, use the Graphviz tred (transitive reduction) tool to
remove redundant edges (such as wwviaudio -> libvorbis):

tools/module-dep-graph.pl | tred | dot -Tpng > out.png && xdg-open out.png

Result:

http://tinyurl.com/64s9r9d

The second picture is much easier to understand.

Looking at the graph begs the question: should CCAN be a collection of
snippets that are easy to use in isolation?  Or should it be a
framework, as the dependency graph suggests it currently is?  Although
the former would be convenient for users, it would lead to maintenance
difficulties (imagine having to fix a bug in multiple copies of ilog).
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