Have the makefile include local.mk if it exists.

This is to allow setting local variables in a persistent way.
Refer to the json-c library as per their examples.

Not using -I makes more sense to me, but it appears that some distributions
put the headers in different locations, and all the example code refers only
to json.h, so we'll follow suit...

Also link against the correct library.
Link correctly against a non-installed libcurl.
Use built-in linking rules (with its own variables).
Merge branches 'build-curl', 'build-jsonc', 'build-localvars' and 'build-implicit-link' into integrate

Conflicts:
	cpp/Makefile
WIP on DNS.
Add CXXFLAGS to the link phase.
Start of test harness.
Return leaf hash.
Temporarily include Eran's https://codereview.appspot.com/66940043/.
Also verify an inclusion proof for a leaf's hash.
Test work in progress.
Get rid of trailing whitespace.
Add missing file, move unmoved file.
Allow binary SCTs and logs on protocols other than HTTPS.
Remaining issues after a rebase.
Fix a couple of stray "https:"s.
Log responses.
Correct path.
Show error content.