AC_HEADER_STDC causes a compile failure with autoconf 2.70

AC_HEADER_STDC causes a compile failure with autoconf 2.70

From https://www.mail-archive.com/bug-autoconf@gnu.org/msg04408.html

> ... the long-obsolete AC_HEADER_STDC, previously used internally by
> AC_INCLUDES_DEFAULT, used AC_EGREP_HEADER.  The AC_HEADER_STDC macro
> is now a no-op (and is not used at all within Autoconf anymore), so
> that change is likely what made the first use of AC_EGREP_HEADER the
> one inside the if condition, causing the observed results.

The implication is that the test does nothing anyway, and due to it
being a no-op from 2.70 onwards, results in the required not being set
to yes, resulting in ./configure to fail.

Change-Id: Ic1ff38d87f791fbf1f2a80512f81bb7110392460
Signed-off-by: Jaco Kroon <jaco@uls.co.za>