configure.ac: Don't check for malloc, realloc

When cross-compiling for Android, configure (erroneously?) decides we don't
have a GNU-compatible malloc, and tries to

    #define malloc rpl_malloc

We don't define rpl_malloc, and it's not clear it would work with the <cstdlib>
C++ header, anyway.  See

    http://nerdland.net/unstumping-the-internet/malloc-has-not-been-declared/

If we aren't using malloc(0), it should be safe to use malloc without the check.