Fix undefined rpl_malloc on autoconf tool in cross compile mode

When using autoconf tool in cross compile environment, sometime there is error like this `undefined reference to `rpl_malloc'`

It is the bug of autoconf tool in the test function AC_FUNC_MALLOC. To fix it we need define an environment variable that forces the test to pass. Define it in the environment prior to calling ./configure and the script will act as if the AC_FUNC_MALLOC check has passed.

export ac_cv_func_malloc_0_nonnull=yes