I think no one really looked into linux build after x64 branch merge.
Some files were added, some were removed, and linux make files remained the same.
I have had a feeling that they do not work for all the time from that moment...
I'm afraid they "need some love" now.
Here is at least the file changes since the time I joined back in April:
EDIT: as for the player movement, you should start at idPhysics_Player::MovePlayer in Physics_Player.cpp, just as grayman said.
It seems to be the main function for all the movement, you can reach all the other code from there.