strtoll ¶POSIX specification:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/strtoll.html
Portability problems fixed by Gnulib:
"0x" and the base is 16 or 0 on some platforms:
Minix 3.3, mingw, MSVC 14.
Portability problems not fixed by Gnulib:
errno
to EINVAL on some platforms. Portable programs should check
for no conversion by comparing str to *endptr, not by
inspecting errno.