improves autoconf check for libdivsufsort

 * We now also check for the existence of the header file.
 * If the lib if not properly installed we now abort compilation
   with an error.
fixes configure script not adding -ldivsufsort

 * closes issue #1
changes to our own binomial coeff. implementation

 * Removes dependecy for GSL
 * Use our own implementation for computing the
   binomial coefficient.