Fix: remove AC_FUNC_MALLOC from configure.ac

AC_FUNC_MALLOC fails cross-compile builds and is
only used to detect non-standard glibc behavior where
malloc(0) does not return a null pointer.

We don't depend on that behavior since we would have
to ship a compat implementation of malloc() for this
macro to be of any use.

Keep it commented because autoscan will report it
as missing and it might get re-introduced.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>