Don't cache ${FFTW}_HAVE_(SIMD|AVX)

Don't cache ${FFTW}_HAVE_(SIMD|AVX)

The error message was incorrect if the library was changed. E.g.
if the library wasn't found on the first try and then (e.g. with
CMAKE_PREFIX_PATH) was found on the 2nd try. Could have been
resolved by using FORCE when setting variables, but that would
have defeated the purpose of allowing the user to overwrite
the value for future versions which might be misidentified.
Any solution which lets the user to overwrite but still works
correctly if the library changes would be difficult. Given that
it is only for displaying a warning message, it is not worth
making it much more complicated.

Change-Id: I0b6c5a204ee0114ef9de9337491a14457917f9d7