tolower
=======

POSIX specification:
 https://pubs.opengroup.org/onlinepubs/9699919799/functions/tolower.html

Gnulib module: ctype

Portability problems fixed by Gnulib:

  * This function cannot be called from plain inline or extern inline functions on some platforms: OS X 10.8.

Portability problems not fixed by Gnulib:

  * On Windows and 32-bit AIX platforms, wchar_t is a 16-bit type and therefore cannot accommodate all Unicode characters.