Fix that check_library_exist is rerun for FFTW

If the library check for fftw plan fails (e.g. missing dependency or
shared library_exists not supported), then the message suggests to change
FFTWF_LIBRARY. But this failed because the check_library test wasn't
rerun because the result was already cached.

TODO: This only fixes it for FFTW. E.g. FindGsl has the same problem.

Change-Id: I170588b0fca2515e9626d7fd85a5730a10777355