The linker is not looking for "constants" it's looking for jump addresses to call functions. Your code obviously uses functions which are not available in the object code. This can have various reasons. Either you did not tell the compiler to use a library which is availabe or (in your case more likely) you did not tell the compiler that jpeg.c
also needs to be compiled for this shared object. But this is difficult to diagnose without seeing the makefile.
Taking a short look at the ndk-modules folder of the git repository I get the feeling you need to build multiple modules. Maybe you just do it in the wrong order. Try building jpeg
first.