Adding functions to libcod made by others

Hi all,

Mitch made some nice extra weapon function and so did filthy. Filthy explained how to add it to my own version in libcod in his thread, but I dont understand it. How to add other people findings to your own libcod version?

EDIT:
Editing the files I understand, but the compiling part is driving me crazy. How to compile on Linux? I searched on google, but it's still unclear

./doit.sh clean
./doit.sh base
./doit.sh cod2_1_3

./doit.sh cod2_1_3 is the name of the lnxded file right?

no, its either cod2_1_3, cod2_1_2 or cod2_1_0