headers/{memory, string}: Relax conditions of declarations `memccpy()`, `strdup()` and `strndup()`

This allows these functions to be called in non-strict-conforming code, and when
`_GNU_SOURCE` is defined, and

* `memccpy()` is available since ISO C23, and since POSIX 2001 as an XSI extension, and
* `strdup()` is available since ISO C23, and since POSIX 2001 as an XSI extension, but
  has been moved into POSIX 2008 base standard, and
* `strndup()` is available since ISO C23, and since POSIX 2008.

Conditions around declarations with `__MINGW_ATTRIB_DEPRECATED_MSVC2005` are
removed, so if someone defines `__MINGW_MSVC_COMPAT_WARNINGS` they get those
warnings, always.

Signed-off-by: LIU Hao <lh_mouse@126.com>
Signed-off-by: Martin Storsjö <martin@martin.st>