[Bug-gsl]extern linkage Visual C++, gsl_precision.h

[mvvenhol]

Under Visual C++ 6.0/W2K builds with /MD option code generation, the following 
external references (1 of 6 shown) in gsl_precision.h cause compiler error 2201:
...
GSL_VAR const double gsl_prec_root6_eps[];

Correction to gsl_precision.h file is:
...
extern GSL_VAR const double gsl_prec_root6_eps[];

M. Venhola

PS Thanks for the Visual C++ port of GSL 1.3