Completely rework configure.ac

Completely rework configure.ac

Full re-organisation of the configure.ac to regroup instructions by
task, automake initialixation, autoconf initialization, OS specific
default values, compiler checks, header checks, linker checks, user
variables, configure switches and so on.

Also BUILT_IN_PLUGINS was moved to --enable-built-in-plugins and
BUILT_IN_PYTHON_PLUGIN_SUPPORT to --enable-built-in-python-plugin-support
as those make more sense as toggles and most users are familiar with
this syntax.

Plus a lot of minor fixes and cleanups.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>