Probe again later if ERT isn't found now

If we write a blank value into the cache, we tell CMake that ERT isn't
found and it won't check again. If we delete it from the cache in case
we didn't find the directories, then CMake will run the find module
again the next time we configure. Just probing for directories is cheap
so this is no problem. This allows us to install ERT, do a reconfigure
and it will then be picked up by the build system.

If we find ERT and it doesn't compile properly, a blank value will still
be written, and it won't try again the next time, so you must wipe the
cache if you *upgrade* ERT into a newer and more compatible version.

A longer explanation is outlined in OPM/opm-core#279.

This pull request supersedes OPM/opm-core#280.