[CMake] Add a helper function to extract paths

Developing CMake code yourself you often need to access path variables like: What is the build directory of dune-grid? What is the cmake modules directory of dune-common? Unfortunately, there are two problems with that:

• The extract method changes depending on whether you are currently configuring the module itself or whether you are configuring a downstream module.
• Normal people tend to forget the naming schemes of the path variable.

This commit introduces a function with a documented interface that allows for querying dune module paths. You give it the name of the dune module, the name of the return variable and a path type. Currently implemented path types are CMAKE_MODULES and BUILD_DIR. More may be added as needed.

I like the idea and the interface.

Added 2 commits:

• 5038ae84 - [cmake] Add a SOURCE_DIR option to dune_module_path
• ce07d980 - [Doc] Correct and enhance the documentation of dune_module_path

Added 2 commits:

• 1980d6c5 - [CMake] Add a SCRIPT_DIR option to dune_module_path
• 37b2df59 - [CMake] Use dune_module_path instead of dune_common_script_dir

Added 24 commits:

• 37b2df59...154cfac7 - 19 commits from branch master
• 4eb8b692 - [CMake] Add a helper function to extract paths
• 125e47eb - [cmake] Add a SOURCE_DIR option to dune_module_path
• a09a4858 - [Doc] Correct and enhance the documentation of dune_module_path
• 2e0f01f7 - [CMake] Add a SCRIPT_DIR option to dune_module_path
• e665cdd8 - [CMake] Use dune_module_path instead of dune_common_script_dir

mentioned in commit 08be7d96

Status changed to merged

mentioned in merge request !86 (merged)

mentioned in commit 110968b5

Mentioned in commit 08be7d96

Mentioned in commit 110968b5