We're not using AC_FUNC_MALLOC correctly; remove it.

When AC_FUNC_MALLOC is used, Autoconf requires that the program provides
a replacement for malloc(), but we don't.
http://lists.gnu.org/archive/html/autoconf/2005-02/msg00142.html

AC_FUNC_MALLOC is only useful if the program calls malloc(0). AFAIK, swfmill
doesn't do this. http://lists.gnu.org/archive/html/autoconf/2003-02/msg00018.html

I'm not sure if this ever works for C++ programs...