FreeBSD portability fix.
As of FreeBSD 11.3 with default C language toolchain, isnan() is broken when compiling with -ffast-math. Workaround: Since floating-point is-functions are not used in the parts of the program doing heavier float crunching, use -ffast-math only there, where nothing breaks. Unbreaks parsing (which relies on isnan()) without slowing down generator. Increase version suffix to "v0.3.1b".