Added missing parts to the CMakeLists.txt code.

[jrutgeer]

As of yet, copy-paste of the tutorial code does not compile.

First error message:

get_target_property() called with non-existent target "gz-msgs::gz-msgs"

This is due to ${GZ_MSGS_VER} not being set.

After rectifying this, another error results:

Missing input file.

This is due to ${MSGS_PROTOS} not being set.

This PR adds these variable definitions to the example code, and also some extra documentation on the use of the ${GZ_MSGS_VER} variable.

[mjcarroll]

Thanks, this should reflect https://github.com/gazebosim/gz-msgs/blob/gz-msgs10/examples/generating_custom_msgs/CMakeLists.txt, but it maybe got out of sync at some point.

[azeey]

Added CMakeLists.txt to gazebosim/gz-cmake#387. Once that's resolved, we should be able to use \snippet instead of copying example code in the tutorial.

[jrutgeer]

Thanks, this should reflect https://github.com/gazebosim/gz-msgs/blob/gz-msgs10/examples/generating_custom_msgs/CMakeLists.txt, but it maybe got out of sync at some point.

Ok, in hindsight that is obvious, but I didn't realise there was an 'examples' folder. I've added a reference to the tutorial so the reader knows where to find the code.

[mjcarroll]

@jrutgeer looks like one minor doctest issue here:

/github/workspace/tutorials/message_generation.md:124: warning: Illegal </br> tag found

[jrutgeer]

@mjcarroll removed the </br>

[codecov]

Codecov Report

Merging #400 (8356265) into gz-msgs10 (25759b6) will not change coverage.
The diff coverage is n/a.

❗ Current head 8356265 differs from pull request most recent head a546ff2. Consider uploading reports for the commit a546ff2 to get more accurate results

@@            Coverage Diff             @@
##           gz-msgs10     #400   +/-   ##
==========================================
  Coverage      97.67%   97.67%           
==========================================
  Files             27       27           
  Lines           1163     1163           
==========================================
  Hits            1136     1136           
  Misses            27       27