Fix GMX_BUILD_OWN_FFTW_URL CMake cache entry type
The type of the GMX_BUILD_OWN_FFTW_URL cache entry was set to "PATH"
and as a result CMake was expecting its content to be a directory.
This commit changes the type to "STRING" and as a result the '////'
workaround in the default fftw URL and custom URLs can now be passed on
the command line which previously did not work as CMake interpreted most
inputs as relative file system paths and converted them to absolute
ones.
Change-Id: I6ed3920d7816b686a031c1a3aee5acd25d7b4692