Fails to build on Unix lacking exp10
Regressed by e9fe7ac7. From error log:
src/format.c:630:12: error: implicit declaration of function 'exp10' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
accum *= exp10( exponent );
^
See also:
https://www.gnu.org/software/gnulib/manual/html_node/exp10.html
https://git.musl-libc.org/cgit/musl/tree/src/math/exp10.c