Add GSL 2.6 support

[leto]

* What is new in gsl-2.6:

** add gsl_vector_memcpy_scale()

** remove -u flag to gsl-histogram

** updated spmatrix module
   - added routines and data structures for all types (float,uint,char,...)
   - added gsl_spmatrix_scale_columns() and gsl_spmatrix_scale_rows()
   - more efficient reallocation of COO/triplet matrices (no longer rebuilds binary tree)
   - enhanced test suite

** add routines for banded Cholesky decomposition (gsl_linalg_cholesky_band_*)

** documented gsl_linalg_LQ routines and added gsl_linalg_LQ_lssolve()