Fix cmake such that Boost::filesystem is exported properly

[jlack1987]

This fixes #166. Ultimately this issue result in the Boost components not getting exported with an ament_target_dependencies and ament_export_dependencies calls.

Before this fix attempting to link against this pkg resulted in,

Target "mylib" links to target "Boost::filesystem"
  but the target was not found.  Perhaps a find_package() call is missing for
  an IMPORTED target, or an ALIAS target is missing?

After this fix it builds properly.

[henningkayser]

Thank you for fixing this. Small nit: could you rename the cmake file to "ConfigExtras.cmake" and also include it to replace find_package(Boost.. )? Mostly for consistency with the other packages and to keep Boost dependencies in a single spot.

[jlack1987]

@henningkayser I went ahead and fixed the file name, but I don't think that file should be included. Is there somewhere else you have seen that does this?

[jlack1987]

Is there any chance this could get cherry picked into foxy/galactic since it's a bug that makes it tough to use this package?

[JafarAbdi]

For some reason, GitHub decided to merge this PR by me only pushing to your branch :/

[JafarAbdi]

@jlack1987 Do you mind testing the latest ros2 branch .?

[JafarAbdi]

Is there any chance this could get cherry picked into foxy/galactic since it's a bug that makes it tough to use this package?

Yep, I'll do right now