Log Message:
sysutils/syslog-ng*: Update to 3.15.1.

## Features

- Support added for `if`/`elif`/`else` blocks to the configuration
  file syntax.
- Dramatically improved debug messages during filter/parser
  evaluation.
- Similarly improved the error messages shown on syntax errors, they
  now show a full backtrace of inclusions, among other things.
- The `hook-commands` module was added, allowing one to run custom
  commands on source or destination setup and teardown.
- Implemented a way to skip processing included config file snippets
  in case a dependency is missing: The `@requires json` pragma.
- Basic client-side failover support was implemented.
- Errors from python destinations are now reported together with any
  exception text (if any).
- `add-contextual-data` gained a new `ignore-case()` option.

## Bugfixes

- Fix a crash that happened on disk queue restart.
- Fixed another crash when a corrupted disk queue file was being moved
  away.
- Fixed a crash that could happen during nvtable deserialization.
- Fixed a crash that occurred when NVTables were stored on low memory
  addresses.
- Fixed an issue with TLS session resumption, the session id context
  value is now properly set.
- We now link directly to the `evtlog` shipped with syslog-ng, and are
  not using the system library, not even when present.
- TLS destinations now work again without `key-file` or `cert-file`
  specified.
- SDATA block names are now sanitized, in order to not break the spec
  when we get our SDATA from sources that are more lax (such as JSON).
- Some internal messages contained key-value pairs where the key had
  spaces in it, this has been addressed, they do not contain spaces
  anymore.
- The STOMP destination will now correctly use template options when
  formatting its body part.
- Fix compilation with OpenSSL 1.1.0
- Fix compilation on FreeBSD.
- Fix compilation on SLES 11.
- Fix compilation on Hurd.
- Fix compiltaion on Solaris 10.
- Fix compilation on MacOS.
- Fixed a value conflict in the `afstreams` module's grammar file.
- Various compiler warning-related fixes all over the codebase.

## Other changes

- POSIX RegExp support was dropped from the filters, PCRE remains
  available.
- Miscellaneous build-system related fixes and improvements (both
  autotools and CMake).
- Update `lib/json-c` to `json-c-0.13-20171207`.