Overhauling autotools setup and some rearranging.

Adding `--without-gsl` option to configure to allow gsl independent
tools to be built and installed without gsl present.

This required some rearranging of the source code and testing code.
I had to move gsl-dependent functions in `partition_combinatorics`
to a separate module `partition_combinatorics_random`.c/h. I also
had to separate the test code for these functions.

I also move gsl-dependent functions in `test_utils` to `test_rng`.