mqtt: only use http-proxy() if it is supported

[alltilla]

Fixes #3765

Alternative fix possibilities, that I could not make work:

• I checked, whether we can use leb version related preprocessor macros from libpaho-mqtt-dev, but I could not find any.
• MQTTClient_connectOptions has a struct_version member and MQTTClient_connectOptions_initializer, which sets it from a preprocessor macro. It seems like it could help, but I could not get useful information from it in compile time.

No news entry is needed, because the http-proxy() feature was not yet released.

Signed-off-by: Attila Szakacs attila.szakacs@oneidentity.com

[kira-syslogng]

Build SUCCESS

[MrAnno]

Thank you very much for fixing this!

[gaborznagy]

I can't test cmake but looks good to me.
(local-built paho-mqtt under /usr/local/lib is not detected by cmake. This is not a regression, I've seen this before this PR. Maybe I'm doing something wrong when compiling paho-mqtt).

[MrAnno]

I think that's okay. You can set CMAKE_MODULE_PATH manually, if you have non-system CMake exported targets.
Alternatively, you can configure syslog-ng to be installed under /usr/local (CMAKE_INSTALL_PREFIX), in that case, all other deps from /usr/local/ will be found automatically.

[kira-syslogng]

Build SUCCESS

[czanik]

I did a fresh compile on openSUSE / SLES using this patch, and syslog-ng compiled successfully both on SLES and openSUSE Leap releases.