This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Some optimized implementations of memcmp() may not start from the
beginning of the string, and may thus segfault.

New in clang 3.6.

According to POSIX and both OSX and Linux manpages,
strcasecmp comes from strings.h, not string.h.

Most OSes also have them available in string.h,
but we just fixed the headers on Haiku and it now
only provides them in strings.h.

We might want to fall back to string.h for other
OSes though...

cf.
http://pubs.opengroup.org/onlinepubs/009695399/functions/strcasecmp.html
http://linux.die.net/man/3/strcasecmp
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/strcasecmp.3.html

Replaces GLib's g_ascii_strncasecmp().

Replaces GLib's g_ascii_strcasecmp().