Linux Build Problems on Ubuntu 20.04LTS

[salderma]

Hi, trying to get this plugin build on my Ubuntu 20.04 LTS laptop.

There seem to be a number of build related errors. I have listed work-arounds with each, but I am not certain if they are sufficient, for proper functionality, though the build is now successful:

1. The setup_ccache script sets config options that do not exist in ccache v3.7.7. I was able to get past this by commenting the inode_cache and file_clone lines from the script.
2. The QT6 Libraries do not appear to be available, except through ppa:okirby/qt6-backports repository.
3. There are 13 compile errors related to -Wenum-conversion which does not appear to be part of Ubuntu 20.04's ccp v.9.4.0. This seems to be remedied by removing the flag from the cmake/common/compiler_common.cmake and cmake/linux/compilerconfig.cmake files.