This is a demonstration implementation of an Oxs_Ext object computing
the demag field using the FFTW library (version 3).

Version 3.x of FFTW, including development files, must be installed to
build and run this module.  In particular, the include file "fftw3.h"
must be installed in either one of the standard places your C++
compiler looks for include files, or else you must modify your
oommf/config/platform/local/<platform>.tcl file to define the location
of this directory, e.g.,

   $config SetValue program_compiler_extra_include_dirs \
         [list "/opt/local/include"]

The fftwdemag module also requires a library with the stem name "fftw3".
On Unix systems the full name of this library will be something like
"libfftw3.a".  On Windows the linker will be looking for a file named
"fftw3.lib".  If the library filename follows a different naming
convention, then modify the oommf/config/platform/local/<platform>.tcl
file to define an alternate naming scheme; for example, if the library
file was actually named fftw3-bc.lib, use

   $config SetValue program_linker_extra_lib_scripts \
       [list {format "%s-bc.lib"}]

As with the include file, if the library file is not in one of the
standard places the linker looks for library files, then modify the
oommf/config/platform/local/<platform>.tcl file to define the location
of this directory, e.g.,

   $config SetValue program_compiler_extra_lib_dirs \
         [list "/opt/local/lib"]

If the library has a dynamic (shared) component loaded at run-time,
then the run-time loader will have to be able to find that as well.
On Windows that means that the corresponding .dll needs to be in a
directory in the PATH environment variable.  On Unix you may need to
modify your LD_LIBRARY_PATH environment variable or take other
measures.

-Michael Donahue, 23-Feb-2014
