Fix Eigen Plugins compilation when linking to grid_map_core

[Ryanf55]

Purpose

Here's a proposed patch for #382. Because the original PR for adding the plugins through CMake and ament is not well documented, I really don't know how this will affect others. Please try this PR out. It will stay open for a while until I get a reasonable number of approvals it doesn't break anything.

Test instructions

• Check this branch out
• colcon build --packages-up-to issue382

And try building your own packages that link to grid_map_core.

[Ryanf55]

This needs to be moved down till after the library is created. You can't call target_compile_definitions on a target until the target exists.

[Ryanf55]

This breaks if we leave it in - any tips on how to test the plugins are working in another package?

[nhewitt99]

Just adding my $0.02 that this PR does fix the Eigen linker issues when using grid_map_core for me, thank you! I can't speak to its potential impact on other plugins, though.

[Ryanf55]

Just adding my $0.02 that this PR does fix the Eigen linker issues when using grid_map_core for me, thank you! I can't speak to its potential impact on other plugins, though.

Thanks for sharing!

[StefanFabian]

My $0.02: This is in line with what I proposed. I would only move the code from the extas file to the main cmakelists and remove the extras file.
If it is not installed, there's no reason to make it an extra file, and with the naming, it could be confusing.

Generally, setting these Eigen compile time variables in the public interface of a library is IMHO bad style as it limits or at least makes it a lot more complicated for people to add their own extensions and makes it incompatible with any other library that does the same (of which I hope there are none but still).
If it's only used in the cpp files, you could set them as PRIVATE and retain the functionality without leaking into downstream packages.

[doisyg]

Thanks for this! I am not sure about the proper solution but this is what I have been end up using to be able to build and use properly grid_map_ros in external packages (in Jazzy):

botsandus@3ce1c30