Thanks Markus. This is very helpful.

When the build system will be fixed, to compile the provided example, a manual build would be done using the following:

cmake -DSlicer_DIR:PATH=/path/to/Slicer-Superbuild/Slicer-build -DIndependentExtension_DIR:PATH=/path/to/IndependentExtension-build ../DependentExtension

This what the extension build system is already doing. See [1] [2] and [3]. We now need to implement the mechanism (and/or provide helper) allowing to create an extension config file.

Thanks
Jc

[1] https://github.com/Slicer/Slicer/blob/master/Extensions/CMake/SlicerBlockBuildPackageAndUploadExtensions.cmake#L127
[2] https://github.com/Slicer/Slicer/blob/master/Extensions/CMake/SlicerBlockBuildPackageAndUploadExtensions.cmake#L209
[3] https://github.com/Slicer/Slicer/blob/master/Extensions/CMake/SlicerBlockBuildPackageAndUploadExtension.cmake#L146

For reference: http://slicer-devel.65872.n3.nabble.com/Linking-an-extension-to-a-separately-compiled-extension-tp4026413p4026439.html

When done, also update the following page: http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Tutorials/CreateLoadableModule

The Config generated by default should expose variable like:

<MODULENAME>_MODULE_WIDGETS_INCLUDE_DIRS

<MODULENAME>_MODULE_LOGIC_INCLUDE_DIRS

<MODULENAME>_MODULE_MRML_INCLUDE_DIRS

// -----------------------------------------------------------
In a nutshell, assuming you have ExtA depending on ExtB, you will have to make sure ExtB provide a config file so that your extension can do find_package(ExtB)

To see how to create a Config file, I would recommend to look at:
https://github.com/davidsansome/python-cmake-buildsystem/commit/3f504d8be5b41086f615494517ddd2cf66f4d365
https://github.com/commontk/DCMTK/commit/f461865d1759854db56e4c840991c81c77e45bb9
http://www.cmake.org/Wiki/CMake/Tutorials/How_to_create_a_ProjectConfig.cmake_file
// -----------------------------------------------------------
Source: Extension dependency discussion - http://slicer-devel.65872.n3.nabble.com/Extension-dependency-tt4028231.html#a4028265

// -----------------------------------------------------------
You could look at: https://github.com/davidsansome/python-cmake-buildsystem

More particularly this commit:

• Export targets from both the installed and build tree [1]
• Add generation of <PROJECT_NAME>Config.cmake [2]

[1] https://github.com/davidsansome/python-cmake-buildsystem/commit/73134d77881ae6648a8f98daaf8630130b7f54ea
[2] https://github.com/davidsansome/python-cmake-buildsystem/commit/3f504d8be5b41086f615494517ddd2cf66f4d365

This was based on the explanation of Alexander Neundorf. See http://www.cmake.org/pipermail/cmake/2013-February/053689.html

We should really update the following tutorial: http://www.cmake.org/Wiki/CMake/Tutorials/How_to_create_a_ProjectConfig.cmake_file
// -----------------------------------------------------------
Source: Discussion on CMake list - http://cmake.3232098.n2.nabble.com/CMakeModules-repository-at-GitHub-tt7583818.html#a7583832

@jcfr Has this been fixed already or there are still things to do?

This is now fixed. @lassoan added support for it.