config: remove unrequired AC_HEADER_STDC

config: remove unrequired AC_HEADER_STDC

Autoconf says:
"This macro is obsolescent, as current systems have conforming
header files. New programs need not use this macro".

Was previously required to ensure the system has C header files conforming
to ANSI C89 (ISO C90). Specifically, this macro checks for stdlib.h,
stdarg.h, string.h, and float.h.

This autoconf option was used to provide conditional fallback support
via defined STDC_HEADERS.

valgrind does not utilize conditional fallback support so, so this macro
is both obsolete and unused, so let's drop it.

Signed-off-by: Rhys Kidd <rhyskidd@gmail.com>