Fix rpl_malloc when cross compiling

Fix rpl_malloc when cross compiling

Work around rpl_malloc being used, see
http://wiki.buici.com/xwiki/bin/view/Programing+C+and+C%2B%2B/Autoconf+and+RPL_MALLOC
for more information:

"The AC_FUNC_MALLOC macro makes sure that the malloc function when passed a zero
argument returns a valid memory block instead of a NULL pointer. This behaviour
conforms to the GNU C library.  Normally, this is a reasonable test that autoconf
makes at build-time. In the case of cross-compilation, however, autoconf cannot
execute a program to verify proper behavior.  It makes the conservative assumption
that the target library will produce non-conforming code.

Failure of this test causes autconf to replace malloc() calls with rpl_malloc()
calls. At link time, if there is no rpl_malloc() function, the linker will fail
with an error describing the missing symbol. The autoconf documentation recommends
adding this harmless code to the application to implement the function."

In Termux the rpl_malloc() usage is useless at best, and may also prevent building
some packages as well as giving runtime crashes for libgc-using packages or others
expecting to intercept malloc.

Previously some packages specified worked around this themselves, but the configure
arguments are now moved into build-package.sh.