Enable parallelisation used of fftw3

[dpshelio]

supersedes #47 as point to the new branch

[dpshelio]

For the records, I manage to compile this if done as follows:

build $ LDFLAGS="-lpthread -lfftw3" cmake [-Dopenmp=on] ..
build $ make

If I use Ninja I get the following error

FAILED: cpp/CMakeFiles/cppurify.dir/FFTOperator.cc.o 
/usr/bin/c++    -I../include -Iinclude/purify -std=c++11 -MMD -MT cpp/CMakeFiles/cppurify.dir/FFTOperator.cc.o -MF cpp/CMakeFiles/cppurify.dir/FFTOperator.cc.o.d -o cpp/CMakeFiles/cppurify.dir/FFTOperator.cc.o -c ../cpp/FFTOperator.cc
In file included from ../cpp/FFTOperator.h:5:0,
                 from ../cpp/FFTOperator.cc:2:
../cpp/types.h:5:23: fatal error: Eigen/Dense: No such file or directory
 #include <Eigen/Dense>
                       ^

I'll play tomorrow with it.

[mdavezac]

With ninja, you have to run 'ninja lookup_dependencies' first

[dpshelio]

retest this please