#35 GSL 2.1 support

[Orion Poplawski]

mathgl fails to compile with GSL 2.1 due to API changes in version 2:

/builddir/build/BUILD/mathgl-2.3/src/fit.cpp: In function 'mreal mgl_fit_base(mglFitData&, mreal*)':
/builddir/build/BUILD/mathgl-2.3/src/fit.cpp:166:25: error: 'struct gsl_multifit_fdfsolver' has no member named 'J'
  gsl_multifit_covar (s->J, 0.0, covar );
                         ^

We're hoping to update GSL to 2.1 in Fedora rawhide soon.

[Alexey Balakin]

It look as it was fixed many years ago