Makes FindErt succeed if ERT is installed after a first configure run (fixes #279 ).

Currently, if ERT is not found during configure, the HAVE_ERT is set
an empty string in the cache. This results in the compile_test of ERT
never being run again (even if ERT was installed in the mean time and
headers and libraries are found).

After this patch FindERT does not set HAVE_ERT manually if neither
libraries nor headers were found. Therfore subsequent configure runs
after installing ERT will succeed.

Fixes OPM/opm-core#279 for me