Add support for FFTW3 and mxpfit

@VictorBaryshnikov has requested to add to mpdroot support for FFTW3 and mxpfit. FFTW3 will be built in double precision and added as standalone dependency. Also our installation of ROOT will be linked to this version of FFTW3 in order to have consistent results. mxpfit will be taken from https://github.com/hydeik/mxpfit.

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