configure: Assume C90 compatible malloc() prototype

This ancient code was attempting to cope with (unknown) systems whose
malloc() prototype was incompatible with the standard.  This test was
fragile; it would break if the build environment provided -Wall in
CFLAGS.

Now that it's 2013, let's assume that target systems have a sane
malloc().  If someone complains, we can revisit this.

https://bugzilla.gnome.org/698716