add CMake macro ament_bump_development_version_if_necessary

[dirk-thomas]

Provide a standardized mechanism to override the exported package number. It can be used when a package makes breaking changes and wants to already identify itself with the upcoming version number to enable conditional logic.

The -dev suffix if a pure recommendation and not strictly necessary.

Regarding the naming of the macro please make concrete proposals if you would like to see a different name.

Generating a C header and providing C macros to compare version numbers is explicitly not included in this PR since it is considered syntactic sugar and not required to satisfy ros2/rmw#188.

[wjwwood]

What happens if the macro is called more than once? We had discussed (offline) an error if it was called more than once with conflicting versions.

In general, the change is ok for me.

I don't think we need a new package for it, since it adds no appreciable complexity nor any new dependencies, so I feel like it could just go in ament_cmake itself...

[dirk-thomas]

What happens if the macro is called more than once? We had discussed (offline) an error if it was called more than once with conflicting versions.

Good question. I was iterating on this while I implemented it. The scenario we considered earlier was two calls being in the code base - potentially in different conditional sections. They might both use the same development version or not. And I think in the case where they don't, the function doesn't have to result in a fatal error. Instead what will naturally happen is that the exported version is the higher of multiple calls. And I would expect that being the intention since each call in a conditional scope "raises the bar" for a certain reason.

I don't think we need a new package for it, since it adds no appreciable complexity nor any new dependencies, so I feel like it could just go in ament_cmake itself...

ament_cmake is a metapackage not containing any functionality so I guess adding it there isn't desired. Imo it certainly doesn't belong in ament_cmake_core. Therefore I think a new package is the right choice. In the future this package can also contain the functionality proposed in #198.

[wjwwood]

And I would expect that being the intention since each call in a conditional scope "raises the bar" for a certain reason.

Ok fine with me.

[dirk-thomas]

Rebased for the necessary Signed-off-by.

[dirk-thomas]

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