ROOT configured with -Dgnuinstall=ON doesn't work when installed with make install

The problem is described in the ROOT forum in the post linked below:
https://root-forum.cern.ch/t/root-master-segfaults-inside-ubuntu18-docker-container/36481

When configured with -Dgnuinstall=ON, CMAKE_INSTALL_SYSCONFDIR is <prefix>/etc/root/, and ROOT ends up looking for the PCH in the wrong location, since it uses a hard-coded <prefix>/etc/ instead for the same directory. ROOT fails to start in this configuration, with the error shown below:

 ERROR in cling::CIFactory::createCI():

  resource directory /usr/local/etc//cling/lib/clang/5.0.0 not found!

error: unable to read PCH file /usr/local/etc//allDict.cxx.pch: 'No such file or directory'

fatal error: PCH file '/usr/local/etc//allDict.cxx.pch' not found: module file not found

The PCH is not found because it's installed in /usr/local/etc/root/allDict.cxx.pch.

To reproduce this issue, one needs to configure ROOT with -Dgnuinstall=ON, build, install ROOT with make install, then try to start ROOT.