CMake: Set RPATHs on Installed Targets

[ax3l]

Set RPATHS on installed targets.
Adding INSTALL_RPATH_USE_LINK_PATH simplifies the usage of openPMD-api when installed against shared dependencies or when building Python bindings while also building the core library as shared (non-default, but possible). Furthermore, we add $ORIGIN (or @loader_path on macOS) to express the internal dependency on our ADIOS1 wrapper libs.

Also: cleans up CMake target C++ standard Control via a function.

[franzpoeschel]

Should we add this to the output of "openPMD build configuration:"?

Also, am I doing something wrong? When I build with openPMD_INSTALL_RPATH=ON, I don't see the rpath, neither with readelf -d /install/location/libopenPMD.so nor with objdump -x /install/location/libopenPMD.so | grep RPATH. I wasn't looking properly

[ax3l]

Yes, it's already printed in the installation summary :)

[franzpoeschel]

Ah, next time I'll look closer