Log Message:
Update gsl to 2.1:

* What is new in gsl-2.1:

** added test suite for example programs

** bug fix when compiling with #undef GSL_DISABLE_DEPRECATED

** bug fix in setting libtool age versioning

** bug fix in gsl_multifit_wlinear()

** added gsl_multifit_linear_rcond() to compute reciprocal
   condition number of least squares matrix

** added gsl_multilarge module for large linear least squares
   systems