the target is now(?) prefixed with Geant4:: otherwise we get a linking error

We no longer test with ROOT 6.22, and in 6.32 PyROOT is coming back but subtly fails on macOS

I hope this will work fine. If the list of headers or linkdefs changes, then cmake should re-run anyway and that will call the file(Generate) to recreate the script, and if only the file content of headers or linkdefs changes, we only have to re-run the shell script.

because add_custom_command does not know the COMPILE_LANGUAGE we are not allowed to treat definitions with that genex inside.
We cannot use add_custom_command(TARGET because that fails with the same error, even though it maybe shouldn't

For compatibility with ninja(?) we need to use these $<CONFIG>$<COMPILE_LANGUAGE> shenanigans

add USE_COMMAND_FOR_GENERATION to fall back to old way

LCIO now exports LCIO::lcio, so we could use that if LCIO is new enough

Dropping DYLD_LIBRARY_PATH produces macOS-only issues when ROOT
libraries are linked with -Drpath=ON. Users of a broken
DYLD_LIBRARY_PATH should be able to unset it manually.

This partially reverts 65a090ff .

DD4hepBuild: add -Wno-psabi to compiler flags to ignore warnings about ABI changes that we will never have issues with

This variable was previously not exported, and the corresponding relaxed python version check was not executed because of this.
This means DD4HEP_RELAX_PYVER was functional in the DD4hep build itself, but not in downstream builds.

Use of nicely named variables such as $p may clash with local user
environments. A similar change was implemented in ROOT a long time
ago:
https://github.com/root-project/root/commit/bba80a143655a4aac63cb5adb2e9f2d73bcd9566

    When using CMake 3.19 or newer and no CMAKE_CXX_STANDARD has been set before the call to find_package(dd4hep), downstream code will error with:
    ```
    CMake Error at /afs/cern.ch/user/v/vavolkl/cmaketest/tmp/CMakeFiles/CMakeTmp/CMakeLists.txt:13 (add_executable):
      CXX_STANDARD is set to invalid value 'CXX_EXTENSIONS'

    CMake Error at CMakeLists.txt:3 (try_compile):
      Failed to generate test project build system.
    ```
    because ${CMAKE_CXX_STANDARD} is empty. I couldn't find any relevant information in the changelog, so this might be a bug in CMake proper. When configuring dd4hep, CMAKE_CXX_STANDARD is assumed to always be set.

As it stands, this project requires to find a python version matching
the one ROOT was build against EXACTLY. This has been discussed in #842.
This adds an option `DD4HEP_RELAX_PYVER` that will accept **any** python
version. It will then perform a custom check if major and minor version
of the ROOT python version and the one found match (in which it should
be safe to assume ABI compatibility). If not, it prints a warning but
continues with the build.

 - including necessary build macro etc

This reverts commit cd161772.

This reverts commit bea8748c.

- Avoid duplicates in paths
- Support whitespace in paths
- Don't add system paths to library paths on macOS