The CC/CXX/LD overrides likely need to use 'gcc-9/g++-9' so it sees the correct version of GCC (assuming that you've installed the gcc-9 package). 19.04 and distros based on it are the earliest that can build AviSynth+ with the version of GCC provided in the main repositories, because support for C++17 (to be able to use the filesystem library) was only added in GCC 8.
That warning/error message in sudo apt-get update was from the Google Earth repository, so no, nothing to do with this.
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