Do not add_library() gmock and gtest if targets already exist

[ojura]

Fixes build problems with Google Cartographer (cartographer-project/cartographer_ros#776), which provides its own FindGMock.cmake.

[dirk-thomas]

And I was just about to ask for the gtest case. But here it is already: 54fdf04

Thank you for the patch.

[ojura]

@dirk-thomas is there a chance you could make a release? CI for cartographer_ros is currently kind of stuck because of this.

[dirk-thomas]

A release especially in ROS distros with a lot of packages is a significant effort (requiring to rebuild all packages across all targeted platforms as well as all users downloading new Debian packages). Therefore low level packages like catkin are not released very often. What ROS distro are you targeting cartographer_ros for?

[ojura]

cartographer_ros targets indigo, kinetic and lunar.

[dirk-thomas]

The gmock support is not part of Indigo (we usually don't backport new features in to older ROS distros) so this change doesn't apply to the indigo-devel branch. You might want to test Indigo separately.

For Kinetic and higher and will try to get this released but it might be a couple of weeks from now.

[tfoote]

@ojura There are other blockers for older targets due to incompatible versions of ceres. cartographer-project/cartographer_ros#158 I believe that there is a goal to get it working on Kinetic if we exclude Jessie, but won't be able to get it onto indigo target platforms. @mikaelarguedas FYI

[ojura]

@tfoote Sure. I am not a Cartographer maintainer though, just a third-party contributor. So, I am not involved much with releasing to ROS. My interest here was to get cartographer_ros's CI unblocked so we can merge PRs which have piled up.

[mikaelarguedas]

There are other blockers for older targets due to incompatible versions of ceres. cartographer-project/cartographer_ros#158 I believe that there is a goal to get it working on Kinetic if we exclude Jessie, but won't be able to get it onto indigo target platforms

Regarding debs:
I haven't tried cartographer 0.3.0 on kinetic/lunar yet, but I expect the protobuf version to be a blocker. to release anything later than 0.2.0 in these distros. I don't think this PR is necessary to release 0.2.0 in kinetic/lunar.

0.3.0 has been released successfully in melodic (ros/rosdistro#17242), but this PR will be required to enable the release of cartographer_ros.

[ojura]

We have just merged a mitigation for this in libcartographer (cartographer-project/cartographer#1011), so it's not a pressing issue anymore for cartographer_ros master :)

[mikaelarguedas]

thanks @ojura for the pointer to the mitigation PR 👍, I'll use that for releasing pending the release of this fix