compare flatorize_c with FFTW

[glathoud]

Just for fun compare on the DFT1024 test case:

• the generic flatorize_c code generation + clang compilation, as well as with GCC (see issue #2 with GCC, but does not always happen)
• the specific FFTW + gcc compilation

misc:

• split dftrealfloat and dftrealdouble for a fairer comparison with FFTW
• in the dftrealdouble case this excludes naive_v8 and flatorize_v8 since they only use 32-bit floats.
  http://www.fftw.org/doc/Precision.html#Precision

[glathoud]

how do we get CPU info (a la uname -a)

clang: remove -g as well

-g: curiosity... which is the other flag(s) that trigger i5 instability

[glathoud]

need to split dftrealfloat and dftrealdouble use cases for a fairer comparison with FFTW