The C89 standard and later defines the `<string.h>` header as part of
the standard headers [1] and on current systems it is always present.
Code included also `<strings.h>` header as an alterinative in some
files. This kind of check was relevant on some older systems where the
`<strings.h>` file included definitions for the C89 compliant
`<string.h>`. Today such alternative check is not required anymore. The
`<strings.h>` file is part of the POSIX definition these days.
Also Autoconf suggests doing this and relying on C89 or above [2] and [3].
This patch also cleans few unused `<strings.h>` inclusions in the libmbfl.
[1]: https://port70.net/~nsz/c/c89/c89-draft.html#4.1.2
[2]: http://git.savannah.gnu.org/cgit/autoconf.git/tree/lib/autoconf/headers.m4
[3]: https://www.gnu.org/software/autoconf/manual/autoconf-2.69/autoconf.html
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