Edit longnum_multinum_sum to use plain array instead of C++'s queue.
Edit longnum_multiply to use new interface of longnum_multinum_sum.
Edit longnum_engine-multinum-sum demo program to use new interface of longnum_multinum_sum.
Opps, I forget to move the num_curr[i] (in longnum_multinum_sum).
Eliminate C++'s #include.
Rename longnum_engine.cpp to longnum_engine.c.

This is to use C compiler. Also, change the final .so compiler to $(CC).
Rename longnum_engine-multinum-sum.cpp to longnum_engine-multinum-sum.c.

Also, change demo-program/Makefile to reflect this.