Build again GSL as static library on Windows/Linux

Turns out GSL < 2.5 uses a special definition file in Windows to export the library as 'GSL', yet the library itself is still called 'libgsl'. This breaks linking of kritaimage and kritatooltransform.

https://github.com/ampl/gsl/commit/a9abf2a65d05cd3e5a9d7286bb1e5ef53b1a4996