Drop md5sum check for GMX_BUILD_OWN_FFTW

Drop md5sum check for GMX_BUILD_OWN_FFTW

The old version gave a confusing error message about a wrong md5sum if
the download failed.

The new version no longer checks an md5sum at all, which avoids the
need to test a CMake version. It also gives an explicit warning and
instructions on how to proceed safely.

CMake bug reported at http://www.cmake.org/Bug/view.php?id=14330
Noted TODO to revisit if that bug gets fixed.

Noted TODO in master branch to show this warning only the first time a
suitable cached variable is set.

Change-Id: I403896505b178251087d71f95362c3754cd4a2de