configure.ac: Added disable switches for features dependending on external libs.

This makes packagers' work easier, because can choose whether they want to introduce new dependency for
their package.
configure: enable-*=yes options make features mandatory.

If a feature is switched on by --enable-feature=yes, then if a
dependency for this feature is missing, configure fails. If it is
switched on by --enable-feature=auto, then if a dependency is missing,
the feature is switched off.
lua: Fixed lua destination queue function parameter

Value-pairs foearch function should be a gboolean and should return FALSE,
if we want to continue.
configure.ac: Added python autodetection feature.

Configure can now autodetect python version choosing from a wide range
of supported versions. If --with-python=force specified, then
configure fails if no suitable python version found. If --with-python=python-<version>
specified, configure fails if the specified version cannot be found.
Merge pull request #64 from talien/f/lua-params-foreach-fix

lua: Fixed lua destination queue function parameter
Merge pull request #49 from talien/f/disable-features

configure.ac: Added disable switches for features dependending on extern...
scl: Add a graphite() SCL

To make it easier to send metrics to Graphite, the graphite()
destination block will set up a pre-configured network destination, with
$(graphite-output) embedded. By default, the payload is empty, and one
must set the options passed to $(graphite-output) when using the SCL
block.

This closes #65.

Requested-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #72 from talien/f/python-version-autodetect

configure.ac: Added python autodetection feature.
Merge pull request #71 from algernon/f/scl/graphite

scl: Add a graphite() SCL
configure.ac: Added default value for with-python
Preparations for 0.3.2
syslog-ng 3.5.5 compatibility

syslog-ng 3.5.5 introduced an incompatible change in value-pairs, in
order to allow time-zone settings to propagate to value-pairs. This
patch ups the syslog-ng dependency, and updates all relevant modules.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
scl: Fixed the elasticsearch SCL plugin

We can't break $(format-json) into multiple lines, as that produces a
syntax error. Changed the elasticsearch SCL plugin to have it on one
single line.

Reported-by: Peter Czanik <czanik@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #74 from algernon/h/elastic-scl-fix

scl: Fixed the elasticsearch SCL plugin
Merge pull request #73 from algernon/f/syslog-ng-3.5.5

syslog-ng 3.5.5 compatibility
Preparations for syslog-ng-incubator 0.3.3

Bumped the version in configure.ac, and updates the NEWS with the bug
fix logs.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>