Fix pkg_config_entry when version number is not specified

[azeey]

🦟 Bug fix

Fixes gazebosim/gz-transport#412

Summary

When gz_find_package is called without a VERSION argument, the <package>_FIND_VERSION variable is empty. But in most of our Find*.cmake modules, we add a pkg-config entry that assumes the version is there.

Example:

gz_pkg_config_entry(GzProtobuf "protobuf >= ${GzProtobuf_FIND_VERSION}")

The pkg-config entry will then be "protobuf >= ", which is invalid and causes the following entry to be treated as a version number.

The fix here removes the operator if the version is empty.

I believe this is why https://build.osrfoundation.org/job/gz-transport13-debbuilder/372/ is failing with

Removing autopkgtest-satdep (0) ...
�[33mautopkgtest [11:28:54]: test build: [-----------------------
�[0mIn file included from /usr/lib/x86_64-linux-gnu/pkgconfig/../../..//include/gz/transport13/gz/transport/Discovery.hh:71,
                 from /usr/lib/x86_64-linux-gnu/pkgconfig/../../..//include/gz/transport13/gz/transport.hh:26,
                 from gztest.cc:1:
/usr/lib/x86_64-linux-gnu/pkgconfig/../../..//include/gz/msgs10/gz/msgs/Utility.hh:24:10: fatal error: gz/math/AxisAlignedBox.hh: No such file or directory
   24 | #include <gz/math/AxisAlignedBox.hh>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
�[33mautopkgtest [11:28:54]: test build: -----------------------]
�[0m�[33mautopkgtest [11:28:54]: test build:  - - - - - - - - - - results - - - - - - - - - -
�[0mbuild                FAIL non-zero exit status 1
�[33mautopkgtest [11:28:54]: test build:  - - - - - - - - - - stderr - - - - - - - - - -
�[0mIn file included from /usr/lib/x86_64-linux-gnu/pkgconfig/../../..//include/gz/transport13/gz/transport/Discovery.hh:71,
                 from /usr/lib/x86_64-linux-gnu/pkgconfig/../../..//include/gz/transport13/gz/transport.hh:26,
                 from gztest.cc:1:
/usr/lib/x86_64-linux-gnu/pkgconfig/../../..//include/gz/msgs10/gz/msgs/Utility.hh:24:10: fatal error: gz/math/AxisAlignedBox.hh: No such file or directory
   24 | #include <gz/math/AxisAlignedBox.hh>
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~

The test uses pkg-config and checking the output of pkg-config --libs gz-transport13 is

-L/usr/lib/x86_64-linux-gnu/pkgconfig/../../..//lib/x86_64-linux-gnu -lgz-transport13 -lgz-utils2 -lgz-msgs10 -lprotobuf

It doesn't include libgz-math7. The contents of /usr/lib/x86_64-linux-gnu/pkgconfig/gz-msgs10.pc from libgz-msgs10-dev is:

prefix=${pcfiledir}/../../../
libdir=${prefix}/lib/x86_64-linux-gnu
includedir=${prefix}/include/gz/msgs10

Name: Gazebo msgs
Description: A set of msgs classes for robot applications
Version: 10.0.0
Requires: gz-cmake3 >= 1.1 protobuf >=  gz-math7
Requires.private: tinyxml2
Libs: -L${libdir} -lgz-msgs10 
Libs.private: 
CFlags: -I${includedir} 

We recently removed the required version of protobuf from gz-msgs (gazebosim/gz-msgs#346) which explains why we haven't seen this failure in gz-transport12-debbuilder before.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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