Ensure `CTKPluginUseFile.cmake` is "installable"

[jcfr]

See https://github.com/commontk/CTK/issues/403#issuecomment-28420747

@saschazelzer As pointed out my @nolden, CTKPluginUseFile.cmake is not yet configured as a relocatable file. Before I start to work on it, I have few questions:

• Is the macro ctkFunctionGeneratePluginUseFile used outside of CTKGenerateCTKConfig.cmake
• Instead of configuring a file named CTKPluginUseFile.cmake, what do you think of having the built-in plugin headers and libraries exposed to the developers after doing either:

find_package(CTK COMPONENTS BuiltInPlugins)

or

set(CTK_USE_BuiltInPlugins 1)
find_package(CTK)

Or even better, we could have a config file generated for each plugin. For example:

• org_commontk_eventbusConfig.cmake
• org_commontk_plugingenerator_uiConfig.cmake
• org_commontk_plugingenerator_coreConfig.cmake

If a plugin org_commontk_b depends on plugin org_commontk_a, after calling:

find_package(org_commontk_b)

it would do the following internally find org_commontk_a which itself would find CTK

Somewhat similar to what is done in Qt5.

[saschazelzer]

I completely agree. First, I would check if I can make the CTKPluginUseFile.cmake file relocatable, just to not start changing too much in the build system. A mid-term solution using individual package config files would be the better approach, as you wrote.

[Warmyone]

I don't quite get what exactly should not work by the explanation but I changed nearly nothing (https://github.com/commontk/CTK/compare/master...Warmyone:relocatable_install), cmake installed ctk, am fetching it from a repo now and have not encountered problems yet.

Should this still be an open issue? And why is the install command excluded from the superbuild setup?