Linker

[petricm]

resolves #118
BEGINRELEASENOTES

• Update compiler flag handling
• Resolve which type of linker is used and assign proper linking flags
• Improve RPATH handling on macOS and add flag LCIO_SET_RPATH default ON

ENDRELEASENOTES

[rete]

@gaede Do you remember what is the default rpath settings in a iLCSoft installation? Do we set this or not?
In other words, what should be the default value here and do we need to update iLCInstall scripts to set -DLCIO_SET_RPATH=ON/OFF?

[gaede]

We always built iLCSoft with rpath: https://github.com/iLCSoft/iLCUtil/blob/master/cmakemodules/ilcsoft_default_rpath_settings.cmake, so no need to specify anything in ilcinstall.
We need make sure, however that the settings in this PR and in ilcutil are compatible...

[petricm]

What is here for the linux rpath follows official CMake instruction on full rpath
https://gitlab.kitware.com/cmake/community/-/wikis/doc/cmake/RPATH-handling#always-full-rpath
the only difference to iLCSoft is that ilcsoft_default_rpath_settings.cmake is missing

# use, i.e. don't skip the full RPATH for the build tree
set(CMAKE_SKIP_BUILD_RPATH FALSE)
# when building, don't use the install RPATH already
# (but later on when installing)
set(CMAKE_BUILD_WITH_INSTALL_RPATH FALSE)

and the system dir treatment.

As for the mac treatment, there is a long story on @rpath here or here, but in short this is way DD4hap and ROOT are built on macOS.

[petricm]

So, if you look in detail, this is the same as it was before with the added option to disable rpath

[petricm]

I was probably not clear. This PR does not change existing LCIO rpath behaviour, it just adds options.