Update Makefile for using with Eclipse CDT; Fix -O0 undefined reference ...

I wanted to read full assembly, but linking with -O0 failed.

    cc -std=c11 -Wall -O0 -mcx16   -c -o main.o main.c
    cc -pthread  main.o lstack.o sha1.o   -o main
    main.o: In function `worker':
    main.c:(.text+0x1c5): undefined reference to `lstack_size'
    main.o: In function `main':
    main.c:(.text+0x367): undefined reference to `lstack_size'
    main.c:(.text+0x3b7): undefined reference to `lstack_size'
    collect2: error: ld returned 1 exit status
    <builtin>: recipe for target 'main' failed
    make: *** [main] Error 1


Found this: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=49653#c11 and added an extern declaration.

I couldn't come up with a better fix.