include/deal.II/bundled added to DEAL_II_INCLUDE_DIRS in deal.IIConfig.cmake even when it doesn't exist

[alexrobomind]

If deal.II is compiled with DEAL_II_ALLOW_BUNDLED=Off (as it is done by the spack package), the include/bundled directory is never created. It is however added to the list of include directories in DEAL_II_INCLUDE_DIRS. This is not a problem when setting up a target via dealii_setup_target, but when I create a dependent library that exports this directory in its INTERFACE_INCLUDE_DIRECTORIES property (which in my honest opinion the targets exported by deal.II already should do), CMake complains with an error message when using that derived library

CMake Error in CMakeLists.txt:
  Imported target "[derived library name]" includes non-existent path

    "[deal.II install directory]/include/deal.II/bundled"

  in its INTERFACE_INCLUDE_DIRECTORIES.  Possible reasons include:

  * The path was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and references files it does not
  provide.

The easiest solution to this would probably to create an empty include/bundled folder just in case, but I don't know where to best put that in the cmake scripts.

[tamiko]

@alexrobomind Will fix it in a minute.

INTERFACE_INCLUDE_DIRECTORIES property (which in my honest opinion the targets exported by deal.II already should do)

We actually merged this changeset already two times (over the last 5 years) and reverted it again due to some older cmake versions having massive problems with it. We might revisit this again for the next release and increase the minimal supported version.