These variables contain the directory paths before the well-known system directories have been stripped out. To support this with pkg-config one has to avoid pkg-config itself doing the stripping by setting PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 and the same for LIBS.

See #2875 for a bit more context, though all ExternalLibraries detect.sh scripts that parse header files are affected.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2903/have-pkg-config-code-path-in-find_lib-set