build: Clean up the main Makefile.am
parser: Always use a valid GError internally

Do not just pass the GError through from the public-facing arguments: we
may want to perform error checking internally at any later date.
parser: Always perform UTF-8 validation

JSON is defined to be encoded using UTF-8, so we should not rely on
the documentation saying so when parsing, but validate the input and
eventually provide a recoverable parsing error.
Add validation tool

The json-glib-validate tool is a small utility that validates the data
found at the URIs passed on its command line.
Add format tool

A simple command line utility to format JSON data; it allows prettifying
and unprettifying JSON.
docs: Add infrastructure to build man pages

The small utilities we build and install should have their own manual
pages.
build: Clean up configure summary
Add abicheck.sh test output to the ignored files