If the files for ERT is not found, then the find module cheats and
indicates that it didn't compile either (since there are some later
tests for HAVE_ERT).
However, erraneously, it gave this variable the value 0, which tests
for false in the CMake code but not in the #ifdef checks in the C++
code. This error is cancelled in opm-core by the fact that it didn't
store it as a cache variable, but resurfaced in opm-polymer when it
probes the second time (as both opm-core as a dependency and
opm-polymer itself declare a dependency on ERT).
It is now stored in a way that should be satisfactory to both
projects.
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