Comment (by Roland Haas):

The --keep-system-libs option is only known starting pkg-confg version 0.7 (from 2012) and at least Ubuntu 20.04 uses an older version (0.29). To avoid this issue I have updated the commit to use the environment variable PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 which has the same effect on new enough pkg-config and is silently ignored by older ones.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2875/handle-system-directories-when-looking-for