We can't find both preprocessing expressions at once, because then
the BUILD_INTERFACE will always be favored if both are present, even
if INSTALL_INTERFACE appears first.
This was affecting the behavior of install(EXPORT) because the
INTERFACE_INCLUDE_DIRECTORIES contained entries like
/foo/include;$<INSTALL_INTERFACE:/bar/include>
As the INSTALL_INTERFACE always evaluates to '0', it always needs
to be preprocessed properly.
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