Use "$(MAKE) -C ..." for running make in subdirectories.

This adds announcements on changes of directory, helping see what happens
better.
Link correctly against a non-installed libcurl.
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.