Avoid using log10 to initialise static data

[rtobar]

This seems unsupported by Fortran (at least 95), and is causing segfaults in gfortran 11 through 14. Compilers shouldn't segfault, even on the face of allegedly invalid code, but let's avoid the issue altogether.

This fixes #13 locally.

[asgr]

Thanks, this worked for me. I've pulled it to my own fork though given Aaron's message re iso not being supported any longer.