added a correlationFFT prototype and calling functions according to the convolveFFT environment.
re-factored the correlation framework, so that the pure correlation itself is implemented by means of an FFTW-Plan like it has already been demonstrated for the convolution. Adapted the tests and the normalized correlation variant to use the new functionality and added zero padding features to the functions.
enabled initMultiArrayBorder() to handle asymmetric borders
introduced VIGRA_SINGLE_THREADED preprocessor flag
made FFTWPlan initialization threaded-safe; refactored FFTWCorrelatePlan and correlateFFT()
replaced detail::setZeroBorders() with initMultiArrayBorder()
cmake: improved configuration of multi-threaded programs