Tick-tock Export Headers

[methylDragon]

This is related to: gazebo-tooling/release-tools#711

Description

ign-cmake dynamically configures an Export.hh file in the public headers of a package. This Export.hh file actually wraps and comments on a lower level dynamically generated detail/Export.hh file that is generated using GenerateExportHeader.

The fact we're using an external CMake library to generate the detail/Export.hh makes it hard to ticktock the lower-level CMake macros.

So instead, this PR defines IGNITION_ prefixed export headers in the configured Export.hh if a target is prefixed with gz that allow downstream users to use the IGNITION_ prefixed versions of the export macros, while leaving those in detail/Export.hh alone (except for IGN_DEPRECATED_ALL_VERSIONS, which doesn't seem to be used anywhere (at least in the core libraries.)

If the library target is ignition prefixed, then no extra macros generate.

This PR replaces all IGNITION_/IGN_ prefixed macros with GZ_ prefixed versions in Export.hh and detail/Export.hh. And provisions IGNITION_ prefixed and the IGN_DEPRECATED macros in the public Export.hh alongside the GZ_ ones, so you can use both!

This means that detail/Export.hh defined macros aren't tick-tocked though! But since they're ostensibly private, it shouldn't be an issue.

Notes

This does nothing new at the moment, but once we start using GZ prefixed macro names, everything will be 🌞 🌈 because of this.

[methylDragon]

Should I just make every EXPORT_BASE point to a GZ_ prefix, and remove the logic?

[methylDragon]

Should I just make every EXPORT_BASE point to a GZ_ prefix, and remove the logic?

I decided to do that. 20748d4

Note that this means every detail/Export.hh will be hard migrated to GZ_ prefixes, with no ticktocking!
I don't see any uses of the macros defined there though, so we should be fine 🤞

[scpeters]

do we want to define IGNITION_ versions of _VISIBLE and _HIDDEN as well?

if so, we should create an ign_export_base variable to use when configuring this file

[methylDragon]

They're defined in here already, using the _ign_export_base variable! 👀

[methylDragon]

Created and populated here: https://github.com/ignitionrobotics/ign-cmake/pull/249/files#diff-955d1f97e2360702839ba9277cc71b9797e65d417d5cb88f383fec0e53ad5124R1447 right above the configure call

[scpeters]

thanks; I could have done a better job reading the pull request changes before making comments like that. I was a little confused on first glance to see all the Doxygen for the redirection macros; I think we could remove them. See my suggested changes in ee17946

[methylDragon]

Changes incorporated!