Don't use AC_FUNC_MALLOC and AC_FUNC_REALLOC

Don't use AC_FUNC_MALLOC and AC_FUNC_REALLOC

We don't expect the GNU'ish behaviour of of malloc() and realloc() and never
implemented the replacement functions rpl_malloc()/rpl_realloc() -- so these
test result in linking failues on systems that don't have a GNU'ish malloc()
and realloc() even though we don't require it!

Introduced by commit 47ad9afc.