Remove obsolescent AC_HEADER_STDC and memcpy check

Autoconf 2.59d (released in 2006) [1] started promoting several macros
as not relevant for newer systems anymore, including the `AC_HEADER_STDC`.

This macro checks if given system has C89 compliant header files such
as `<string.h>`, `<stdlib.h>`, `<stdarg.h>`, `<float.h>`,... and defines
the `STDC_HEADERS` symbol [2]. Case is that current systems should be
well supported with at least C89 standard headers [3].

Given headers are still additionally checked with the `AC_PROG_CC`
macro, yet not needed anyway.

Additionally, the HAVE_MEMCPY check has been removed. The memcpy
function is standardized by C89 and later.

Refs:
[1] http://git.savannah.gnu.org/cgit/autoconf.git/tree/NEWS
[2] https://www.gnu.org/software/autoconf/manual/autoconf-2.69/autoconf.html
[3] https://port70.net/~nsz/c/c89/c89-draft.html#4.1.2

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