This works around older CMakes that refuse to set arbitrary
properties on a target. Instead, use calamares::kpmcore
as the specific indicator that kpmcore was found.
Breaks build of libcalamares, since that needs **something**
regardless of whether KPMcore was found.
- the alias libraries calapmcore and calamares::kpmcore
are always created; if there is no KPMcore, they handle
definitions to signal that.
- upstream kpmcore target is the one we should be testing
to see if KPMcore itself is there (or, use KPMcore_FOUND).
- kpmcore (when used as target "kpmcore") has an interface
include directory that does not contain the "kpmcore/"
subdirectory. But the headers it has installed, assume it
is there (e.g. kpmcore internals use #include <fs/filesystem.h>).
- add an alias at Calamares level that sticks in some more
includes, adds the relevant WITHOUT_kpmcore when it's
not there, etc.
I think we had this (kind of) module a long time ago and it was
removed for over-complicating things; re-introduce one now that
KPMcore is used in 3 different places and all would benefit
from consistent API handling / defines.
