Add CMAKE_PREFIX_PATH to cmake-gui

Add CMAKE_PREFIX_PATH to cmake-gui

CMAKE_PREFIX_PATH didn't show up in ccmake, because it wasn't in
the cache by default. Given that it is the easiest way to specify
external dependendencies it should show up. Also prior one had to go
to the advanced settings to specify the path to FFTW. Modified
the FFTW message to explain that CMAKE_PREFIX_PATH is also an
option. This makes it possible to set it (if it isn't found
automatically)without having to open the advanced settings.

Change-Id: Ib36ab004aac8f65d2a145d67b72b8c8f4fa065c8